J.B.
I would do what he wants to do -it's his birthday! Depending on the activity, if he wants to do something with expensive tickets, let him know how many friends you can afford. If he just wants a sleepover, 8-10 should be fine. I have no problem with big parties for any age as long as the kid wants that and the party is the main attraction. If my child wants to go to an amusement park or something -then I'll say "Limit it to ------ amount."