Originally Posted by Rejakor
You appear to be referring to Chapters of space marines, i.e. post-heresy legions broken apart by the changes made by rowboat girlyman (roboute guilliman) in the Codex Astartes.
Pre-Heresy Legions varied wildly in size but the average was 'about 10,000'. So depending on the specific size of the 20 Chaos Legions, it could have definitely involved hundreds of thousands of space marines AND daemons.
There were 9 Chaos Legions, not 20...I have no idea where you got that number.
And yes, I was referring to Chapters, because the Lexicanum says that 50 Chapters or contingents of Chapters have gone rogue since the Heresy...so the forces of Chaos have had an absolute maximum of 50,000 reinforcements, and it's doubtful all of them survived up to the current day. Only one of the original Traitor Legions - the World Eaters - were present on Armageddon in force, led by Angron the Demon Primarch.