Human tribes are typically about 150 ppl. At least, that’s what we evolved for, I’m sure there’s some flexibility on that number. Beyond that the constraints of “who’s who” becomes hard to recall for an individual, weakening social bonding.

Anything beyond that is technically social engineering of shared identity. Up to you to decide what level of manipulation (good/bad) that entails. In short, we aren’t made go innately understand million+ scales