I think he wants to know how many people can play on a server run by a machine in use before it starts to lag, in which case he would have to set up a dedicated server.
If so, then it really just depends on the hardware in the host machine, and especially on what game you're playing. We've run 20+ people in a Counter-Strike server on a machine in use (1.1 Athlon, 256 sd133, Geforce2).
Unless its a really high demanding game on a slow system, you should probably be ok.