Ahh, here is from their FAQ:
Approximately 10,000 players per world consisting of 100 player district maps. A hundred customized players driving unique cars, doing crimes, shooting or busting each other is well worth checking out.

So it would seem that the world itself can have 10k. Then there are 100 instances or zones that can hold 100 people. I am guessing that you can go from zone to zone as long as there is room. So that is massive, just with limited movement.