Strictly, they aren't limited to Chapters- small groups of Marines (companies etc) have gone renegade as well.
Some Legions have demonstrated the ability to implant new recruits (Iron warriors for example)- but I'm not sure if this applies to the World Eaters.
After Istvann III there were on the order of 80,000 World Eaters according to the Forgeworld book "Horus Heresy: Betrayal" (legions averaged 100,000, not 10,000)
By the 41st Millennium there's probably somewhat less- even with them recruiting renegades.
Kharn shattered the World Eaters Legion into warbands at some point not too long after the Heresy- so it's possible Angron didn't have access to the whole Legion- just many of its warbands.
All in all I'd say you're probably looking at a few thousand World Eaters- but a lot less than 100,000, on Armageddon.