Grand Theft Auto Online Revealed - IGN
"A dynamic and persistent online world for 16 players."
16 is pretty big. I wonder though, is it 16 players at a time? Or can you have 30 people in that "world" as long as not all 16 are online at the same time?
I also think its really cool that they did extra voice acting just for missions online. That's really crazy.
In a way, I'm glad the online isn't available immediately. I like to finish the single player mode first before jumping into online stuff anyway (in case of spoiler related missions/locations).