9 is the sweet spot, with room for 10.
You need a bit of extra room for working on a machine, and/or moving a new one in before one is moved out.
Having 9 also allows for enough games to hold small/mid size pinball tournaments (8 for the tournament, and 1 extra in case a machine breaks down just before or during the tournament).
9 machines is definitely enough machines for friends & family to play when you are throwing a party.