The ancient City States of Renaissance Italy or Ancient Greece (much different animals than the overpopulated cesspools of the East) had a population of several thousands (say five to thirty thousand). Space exploration might force this new dynamic as populations of these similar numbers venture out, never to return to earth, with little to no newcomers, and communications no longer being instant with earth (a dynamic similar to that of ancient empires).
While I don’t believe these are likely to be independent from Earth at first, there will likely be a disconnect from Earth as established colonies claim independence IF they’re able to sustain themselves without earth comfortably. Also, rouge megaships are likely to come into existence and eventually lose contact with earth, thus gaining their independence as a city state.
This is assuming that the human race exists and doesn’t merge with machines of course…