First Founding

"They shall be my finest warriors, these men who give of themselves to me. Like clay I shall mould them, and in the furnace of war forge them. They will be of iron will and steely muscle. In great armour shall I clad them and with the mightiest guns will they be armed. They will be untouched by plague or disease, no sickness will blight them. They will have tactics, strategies and machines so that no foe can best them in battle. They are my bulwark against the Terror. They are the Defenders of Humanity. They are my Space Marines and they shall know no fear."

--The Emperor of Mankind on the creation of the Adeptus Astartes

After he conquered Terra, the Emperor of Mankind set out on a Great Crusade to re-unite the lost human colonies into a mighty Imperium. He soon realized that bog-standard humans wouldn't be enough; some situations called for a small number of devastatingly powerful warriors, a surgical strike force to smash enemy linchpins. To fill this role, he created his Primarchs, and from their genes, he created the Legiones Astartes, the twenty Space Marine Legions:

Legion Number	Primarch	Homeworld	Name of the Legion	Allegiance
I	Lion El'Jonson	Caliban	Dark Angels	Loyal?LOYAL
II	+++Records expunged+++
III	Fulgrim	Chemos	Emperor's Children	Traitor
IV	Perturabo	Olympia	Iron Warriors	Traitor
V	Jaghatai Khan	Chogoris/Mundus Planus	White Scars	Loyal
VI	Leman Russ	Fenris	Space Wolves	Loyal
VII	Rogal Dorn	Terra, Inwit	Imperial Fists	Loyal
VIII	Konrad Curze	Nostramo	Night Lords	Traitor
IX	Sanguinius	Baal	Blood Angels	Loyal
X	Ferrus Manus	Medusa	Iron Hands	Loyal
XI	+++Records expunged+++
XII	Angron	Nuceria, Bodt	World Eaters	Traitor
XIII	Roboute Guilliman	Macragge	Ultramarines	Loyal
XIV	Mortarion	Barbarus	Death Guard	Traitor
XV	Magnus the Red	Prospero	Thousand Sons	Traitor
XVI	Horus Lupercal	Cthonia	Luna Wolves/Sons of Horus	Traitor
XVII	Lorgar Aurelian	Colchis	Word Bearers	Traitor
XVIII	Vulkan	Nocturne	Salamanders	Loyal
XIX	Corvus Corax	Deliverance	Raven Guard	Loyal
XX	Alpharius Omegon	Unknown	Alpha Legion	Traitor/Loyal/Maybe Both

Organization

The Legio Astartes, being far larger than current Space Marine Chapters, were organized along drastically different lines. At the head of the Legion was its Primarch Commander who oversaw the Legion's headquarters. Below this were the Chapter of the Legion, each of which was commanded by a Lord Commander. Each Chapter in turn was divided into a number of Battalions led by a Lieutenant Commander. Each battalion was then further divided into Companies led by a Captain. The Company, itself divided into a number of Squads, was the basic military division of the Legio Astartes. Due to the varying sizes of each Legion and the varying nature of supply and attrition, there was no fixed number of how many Chapters, Battalions, Companies, and Squads a Legion would contain. But even then, there was no fixed institution that a particular Company should be composed of certain units, with each Company specialising naturally as marines and officers became veterans of a certain form of warfare.

Number of Legionaries

The Legions were massive armies, and the size of each could vary tremendously. A precise number was never truly achieved and maintained. Even during the Great Crusade, some Legions were very numerous, while others were not. The numbers would always vary with new recruits and inevitable battle-losses, and also important was the availability of potential recruits and the administrative skills of the Primarch and his officers.

The most numerous Legion of all was the Ultramarines. The Thousand Sons of Magnus were of a small number as many of them had developed mutations or uncontrollable levels of psychic powers. Fulgrim's samples had been largely lost, and this left the Legion of the Emperor's Children also with a very small number. Both of these Legions would increase their numbers to acceptable standards only after their Primarchs were found.

The approximate sizes of a few of the Legions at the start of the Heresy have been given in various sources (it should be noted that originally these numbers were much smaller, a full zero knocked off until they were upped by the BL/Forgeworld writers):

• Ultramarines - 250,000
• Dark Angels - 166,000 to 216,000 (They were considered the largest legion, the number fell below 166,000 and the Ultramarines overtook them)
• Iron Warriors - 150,000 to 180,000 - (They also had the largest / most powerful starships)
• Sons of Horus - 130,000 to 170,000
• World Eaters - 150,000
• Alpha Legion - 90,000 to 180,000 and possibly even more. No-one knows for sure.
• Word Bearers - 100,000 to 150,000
• Blood Angels - 120,000
• Night Lords - 90,000 to 120,000
• Iron Hands - 113,000
• Emperor's Children - 110,000
• Imperial Fists - 100,000 - (They also had the single largest fleet in the Imperium, consisting of 1500+ vessels plus the Phalanx)
• Death Guard - 95,000
• Salamanders - 89,000
• Raven Guard - 80,000
• Thousand Sons - 10,000

Other Legions have not yet been so explicitly numbered, but their impression can still be gained by how they are described.

• Space Wolves - Were considered to be smaller than most of the other Legions, but a pre-heresy skirmish with Angron showed that Leman Russ knew how to obtain his objectives extremely efficiently.
  • Considering that Russ divided his Legion only once following the Heresy then kept his "Chapter" at a relatively stable size after that we can figure that the number of original Space Wolves is close to 6000 or more, which when combined with Custodes & Sisters of Silence would be a reasonably fair match against the 9000 or so Thousand Sons at the battle of Prospero. Following that battle, when Bjorn is waiting for a new arm, the Forge master explains there are 4000 other warriors in the fleet who are also waiting for his services; and to get in line.
• White Scars - Fuck knows, in the books Scars/Brotherhood of the Storm, one of the human characters from the Departmento Munitorum is attached to the Legion to figure precisely this out, but finds it practically impossible due to the way that the legion operates.

Horus Heresy

For all their power, the Space Marines and Primarchs were not perfect; half of them were sufficiently flawed to be tempted by (or driven to) Chaos, led by Warmaster Horus. The traitors were defeated, but at a terrible cost, including the near-death of the God-Emperor of Mankind and trillions of deaths.

Roboute Guilliman decided that the so-called Horus Heresy was proof that one man could not be trusted with power over one-twentieth of the Imperial Armed Forces, so he enacted several reforms to divide the Imperial Army into the Imperial Guard and Navy, and split the remaining loyalist Legions into Chapters, in accordance with his Codex Astartes. This event was later known as the Second Founding.

Legions in 40k

Technically all of the Chaos Marines are still 'legions', at least in name. They never split into chapters so legions they technically remain. However, since they are all split up into roving warbands and there is no central control it's hard to say they really count anymore. Perhaps most importantly it's impossible to guess how many dudes there are even technically in any legions pool. When they aren't fighting the imperium they are fighting each other, and since none of the Black Crusades have been successful we can reasonably assume that marines aren't pouring out of the Eye Of Terror in their hundreds of thousands. It's utterly impossible to guess how many dudes they got. They got some. Probably not that many. Definitely many many many less than they arrived at the Eye with, which was obviously rather fewer than they began the war with.

As for the loyalists... Since the introduction of the Codex Astartes, there are obviously no longer any Space Marine legions though that does not stop certain chapters from being described or implied to be at "Legion Strength". Though this is often a common Neckbeard error and sometimes even gets used in the fluff, where the term is mistakenly used to describe any significantly oversized chapter, though even then they never compare to the forces from the Great Crusade. Though some make a good attempt at it:

• Black Templars are obviously over-strength in multiples of 1000 and keep to the spirit of the great crusade unlike any other chapter. This force is made up of three primary crusade fleets of unknown size which can end up spread over up to thirty engagements, where the size of a force could be as large as "nearly" chapter strength (but not over for obvious reasons) or as small as a single ship and a handful of squads, so can be averaged at around chapter strength for each primary fleet. This arrangement is meant to be fluid, and changes all the time as fleets merge and disperse. Respectable figures put their numbers between 2000-6000
• Space Wolves make no secret of the fact they ignore the Codex Astartes, but even then they are no-where near their original size. In M32 during the Battle of the Fang it is remarked that most of The Aett is actually abandoned. Furthermore, Logan Grimnar's own Great Company numbers 200 marines prior to the Sanctus Reach campaign and is the largest company on its own (ie: not including the chapter assets). Their current number aught to be between 1200-3000.
• Astral Claws were actually accused of Legion building and declared heretics even when they were not yet followers of Chaos, their number at the time of the Badab War was around 3000, though they never even used that number as a single force and spread themselves throughout their PDF forces
• The Unforgiven are most likely one of the closest examples of a true Legion in that each of the successor chapters arrange themselves under one secret command structure (the Inner Circle). Though they each obey the letter, if not the spirit of the Codex Astartes and maintain mostly standard chapters of ten companies. The combined numbers of Dark Angel's successors would put them at around 150,000 marines, and this is just a rough estimate, though we don't know if all of the Dark Angel's successors actually consider themselves part of the Unforgiven. Some definitely do though. Oh yes indeed. The Inner Circle has been known to send a random successor chapter across the galaxy to do the space marine equivalent of getting them a six pack (of traitors) when the main chapter is too busy.
• Ultramarines - Ironically, the ones who uphold the Codex Astartes most strictly are the most hypocritical when it comes to deviations from it. The 23 Second Founding chapters of the Ultramarines are all represented in the senate of Ultramar and look to the Lord of Macragge as their nominal leader. The Ultramarines even keep a SPARE chapter lying around to replenish their own ranks whenever they suffer terrible casualties. Also if certain people had their way then EVERY chapter would answer when the Ultramarines snap their fingers. Because the Ultramarines account for nearly half of the later-founded chapters in the Imperium, they would amount to around 500,000 marines.
  • Then again... The thing is that all of the original legions maintain ties with with their subsequent founding legions. Blood Angels occasionally call all their bros together, and last time they did everyone out to the 23rd founding showed up for blood and pancakes. Every now and again every Dorn-descended chapter sends a champion (and his mustache) to a big fighting contest to see who is the Dorniest. Essentially every Legion that wasn't murdered into damn near extinction (ie had enough marines to have many descendants) maintains some tie between them. Ultramar spanned FIVE HUNDRED worlds. That's a lot of worlds. And all of them were tithed to the Ultramarines. Guilliman wanted his boys to be warrior-kings. That meant that he gave worlds to be run by marines, and in the end his ideal was each chapter master and senior captain would be master of a world too and look out for it's population. Sooo... 500 worlds -> 500,000 marines, 1 modern chapter per world, not exactly shocking when you say it like that. And let's not to forget that Ultramar was a republic (In Plato's sense of a republic anyway; ruled by just men (which is different to being ruled just by men by the way) as opposed to democracy specifically). So there is a senate. Because that's what republics have. But the senate isn't something that secretly orders chapters around (*ahem* Dark Angels) it's just what there was around when each chapter master split off into his own chapter instead of being part of the legion. In short - The Ultramarine decedents are bros. They bro it up. They probably bro-fist Papa Smurf since he has the big giant pimp-gloves. I know Matt Ward fucked the reputation of the Ultramarines, but he wrote almost none of their fluff (seriously, he said a bunch of shit outside of canon but other than a few pages here and there in codexes he really hasn't effected all that much, he's just effected how you feel about them which isn't the damn same) and they definitely don't do anything that other 1st founding chapters aren't doing. Except they do it bigger.

Two Unknown Legions

In the First Edition of Warhammer 40,000, the twenty First Founding Chapters were all known. It seems that Games Workshop had more love for some than others; when they made fluff revisions in the transition to Second Edition, the Valedictors and Rainbow Warriors were demoted to one of the "Later Foundings."

Nowadays, nothing concrete is known about the Legions II and XI, and likewise their unknown Primarchs. Officially, the Imperium deleted all records regarding the "Lost Legions"; the only reminder of the two legions were empty plinths in the Hegemon where statues of the Primarchs stood at the Imperial Palace. Throughout the Horus Heresy series, it is suggested that the Space Wolves destroyed them for some reason. The Horus Heresy: Massacre includes a timeline of the events at the end of the Great Crusade, and in 965 and 969.M30, the Space Wolves engaged in two missions from which all data was redacted. In the book The First Heretic, when a daemon takes the Word Bearers on a trip back in time to show them the creation of the Primarchs, the Word Bearers dialogue indicates that the XI Legion in particular did something bad enough for the Emperor to lead their purge himself, and that the left-over Marines were folded into the Ultramarines (their Chaplain dismisses it as a rumor, though it is public record that the Ultramarines received a major boost in troops at the time). In the book Fear To Tread, Sanguinius tells Horus that he hasn't revealed the existence of the Red Thirst to the Emperor because he fears that the Blood Angels would be purged as well, indicating that gene-seed flaws may have also been a factor. Deliverance Lost has a dialogue between taking place during Corax's first meeting with the Emperor where he asks why only sixteen of his brothers were waiting to meet him if he was the nineteenth Primarch to be found, only for the Emperor to deflect the question; consequently, we can assume that the Legions were purged sometime before Corax's discovery (and were never around for the Horus Heresy). He also forced the remaining Primarchs to swear an oath never to speak of their absent brothers, so whatever they did must have been extraordinarily bad. In Legion, a ship's captain notes that the naughtiness of the Alpha Legion isn't the first time that a Legion has "overstepped it's mark" and that the Imperial Army fleet should report the Alpha Legion "before they become too powerful," which might imply that one of the Legions got away with naughtiness for a long time and then used their entire Legion to fight the Emperor. 'Horus Heresy Book 3: Extermination also hints at the fate of the two missing Legions, first hinting that one Legion did not pass the "Alpha" intake (to take them up to Legion-Strength) when going through testing and the other saying that "the disaster of [REDACTED] had proven the folly of attempting to recruit Legiones Astartes stock from potentially tainted sources." These could be in reference to the same Legion (using tainted stock in the Alpha intake) or two separate ones, with Legion A fucking up in the Alpha intake and Legion B using tainted stock in the later Great Crusade.

Since Horus and the other traitor Primarchs managed to keep their records un-expunged during the Horus Heresy (and afterwards, since the Space Wolves clearly know who Magnus is, the Grey Knights clearly know who Mortarion is, and Horus' name seems to have become a swearword), it can be assumed the missing Legions' naughtiness was something even worse. Possibly they refused to wear pauldrons, we don't know.

Or there may simply be no missing Legions. According to a source who allegedly worked for Games Workshop for five years (seen here), Rick Priestley (who helped write the first Rogue Trader) read about the Roman notion of Damnatio Memoriae, and simply threw in the idea of two "missing" Space Marine Legions as a "nod" to Imperial Rome (specifically, the three Roman legions whose numbers were never used after they were wiped out in the disastrous Battle of the Teutoburg Forest. So, nothing significant can ever be revealed about them, because they never really existed in the first place. Except they totally existed in the current canon, even if they started as a joke. Also, the real reason that they were kept around between Rogue Trader and recent revelations was so that people could make up their own pair of Primarchs for model painting and personal fanon (although this has lost its effect as doing so is a step in sending them down the path of the Mary Sue).

The Traitor Legions and Warbands of Chaos
Chaos Legions:	Alpha Legion - Black Legion - Death Guard Emperor's Children - Iron Warriors - Night Lords Thousand Sons - Word Bearers - World Eaters
Legion Offshoots:	Apostles of Contagion - Bloodborn - Broken Aquila Foresworn - Mouldering Claw - Plague Fleet - Prodigal Sons The Consortium - Warband of Subsector Aurelia
Fallen Chapters (Including Judged):	Adharon's Reavers - Blood Gorgons - Company of Misery Corpus Brethren - Crimson Slaughter - Deathmongers Death Shadows - Invocators - Lords of Decay Oracles of Change - Red Corsairs - Shriven The Brazen Beasts - The Flawless Host - The Scourged Skyrar's Dark Wolves - Steel Cobras - Voidrippers
Unknown/ Other:	Apostles of Minthras - Claws of Lorek - Disciples of Destruction Dragon Warriors - Extinction Angels - Hakanor's Reavers - Punishers The Cleaved - The Purge - The Pyre - Sons of Malice - Sons of Vengeance The Reborn - Violators - Warp Ghosts - Bleak Brotherhood