Within the context of the US, tribe can refer to a specific group of Native Americans. Group size is irrelevant, e.g. the Navajo Nation has the largest reservation and 165k members.

Each tribe is free to determine its own legal structure, as it is a separate polity (a sovereign nation) from the US -- sort of. US federal law still applies on federal reservations.

https://en.wikipedia.org/wiki/Native_Americans_in_the_United...