system
Given the following post, respond as if you're commenting on Reddit:
human
[Warhammer 40k] How do the Chaos Space Marine Legions replenish their numbers?

I know that many of the original warriors from the Horus Heresy still reside within the Eye of Terror. However over 10,000 years many factors such as the numerous failed crusades against the Imperium, Chaotic infighting, the extreme physical conditions and the murderous nature of the climb upward through the ranks would whittle down this core of ancient warriors. How do the different legions go about recruiting and training new warriors?

Some legions like the World Eaters seem to be in permanent decline and the remaining warriors who do not ascend to demonhood will likely eventually be wiped out over the next few millennia of warfare. For those legions where some vestige of organisation remains from where and how do they recruit and train new warriors?
gpt
From Lexicanum:

"The Chaos Marines still maintain the process of gene-seeding - the transformation of humans into superhumans through organ implantation and associated psychological and chemical conditioning - in order to create new Chaos Marines. The children that are turned into new Chaos Marines are bred from slave stock and captives acquired from raids on Imperial worlds. The process is a brutal ordeal, differing from the carefully measured program of development used by Imperial Space Marines.

In every Legion, Apothecaries, or their Chaos Marine equivalents, are still charged with the important task of retrieving the gene-seed organs from their fallen brethren. "

In addition, it was mentioned in fluff somewhere that the thousand sons rubric marines (the golem like ones) can be re-summoned into their repaired armor, but obviously that doesn't apply to your standard marine.

It's also worth pointing out that Fabius Bile has claimed to have uncovered the secrets to the primarchs, and astartes.  His continued existence could be because the Chaos Legions need him to keep reproducing themselves.