Originally Posted by Rejakor
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.
Indeed, chaos marines count reinforcments not only from renegades but also from regular humans that happen to worship chaos, prove their worth and are lucky enough. Fabius Bile has something to say about this:
Each of the Legions has now nominated aspirants seeking to throw themselves upon our mercy in the vain hope that we may deem them worthy to join our ranks. Those loyal to the shrunken corpse on Terra still cling to their own processes by which perhaps one in a hundred neophytes may survive to become a battle brother. The methods I have developed over the last millennia are more stringent, for we must be pure in our hatred and hard of heart, body and soul. Fewer than one in every thousand survive, and I strive each day to lengthen these odds still further.
So that's 1 in each 1000-10 000 chaos cultists steping forward that rise as a new chaos marine. Considering how cultists usually rival IG in sheer numbers as whole populations revolt against the hated IoM, that's not shabby at all!
In one of the Iron Warrior novels they go raid a secret deposit of loyalist gene-seed to greatly swell their own numbers.
Plus in the latest chaos codex, the World Eater warbands are described as having chirurgeons specialized in turning new recruits in axe-crazy berzekers, so they've also been keeping up their own numbers.