I dont think there is an actual cap on the number of NPC's.
Maybe it's just you. Have you flossed? Seriously though, the mechanics of the game go beyond the realms of the imagination sometimes.....like how many bases have been destroyed in all the missions ever done by players..but look closely - they have names, these NPC's. I do occasionally patrol with npc's, which is weird, I know, but instructive. They actually do spend time floating around doing nothing. No wonder they get all excited when you turn up.
