Originally Posted by The Glyphstone
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.
Chaos controls various areas and the chaos space marine legions recruit from worlds in those areas, much as space marines recruit from worlds in imperium controlled space.
The imperium has more space that it controls, but it doesn't have access to daemonic resources and so has to spend more time and energy fighting off things like necrons and orks.
But chaos fights itself more than the imperium, so it's a bit of a wash.
That said apparently the Eye of Terror is a very large region of folded space and daemon-worlds, so the actual amount of chaos space marines appears to be 'any'.