Back in The Sims 2, I used to throw a party with the maximum number of guests, then pick up the phone and "Invite Over" every other Sim in the phonebook. I haven't tried it in The Sims 3 yet, but I presume you could still boost your numbers that way. Using a public lot would also work as other townies would wander onto the lot during the party. Otherwise, I think you can only invite 10 or so, as Tallstar said.