I'd say, considering how big the city is, they could begin with around 20/50 (maybe in between both cities, and migrating as time goes on, in order not to flood the experience and early days). Considering how easy you can die, i'm imagining that number would decrease rapidly as time goes by, maybe even factions could be formed, who knows? I still retain the memory of the demo in which i could see a lone survivor running around with a shotgun trying to make it into the police station, only to get himself caught by even more zombies than he killed initially. Stuff like that could add a lot to the game, i can't even imagine how awesome it would be to find a survivor after like 5 months of gametime, they could havefer you shelter if you manage to find certain items within the city, or could turn hostile if you stay too close to their fort..... The possibilities are endless, and seeing how they pulled off stuff like Bleach, the Luck factor and many others, i definitely don't see this as a wild idea, perhaps it could even be added later as a mod.