^As I stated, it's impossible that they'd been there for millennia. The only credible possibility is that they were taken in the 17th or 18th century. This is something too many people overlook: That the Preservers are a group from modern
times, not ancient times or prehistory.
But yes, there would have to be multiple population centers. We're talking about cultures that were at least semi-migratory, and from the look of their dwellings, that hadn't changed in the centuries since their abduction. It's reasonable to expect that they branched out into multiple bands/villages of a size that was sustainable for that kind of subsistence pattern, spread out far enough that each had enough territory to provide necessary resources.
Also, we know they had fighters, like Salish, so it follows logically that they had rivals. Like their ancestors, they probably had periodic raids or warfare between communities, probably took hostages from rival communities and adopted them into their own to take the place of individuals lost to combat or disease, etc.