255 netgames
255 players per netgame

65535 players in all ... although i wouldn't go above 64players in each, you could use each netgame as a "zone" as it were and make your own multizone.
[edit]
forgot to mention that this is all per IP...
each IP can have upto 255 netgames, with upto 255 players each.
an mmorpg server can generally use upto 255*255*255*255 -1 IP addresses for its game zones on because of internal servers (and DNS Gateways)
effectively the limit is how you program the server, which isn't a barrel of laughs or simple :p