Depending on which expansions you use and how much space you have, 1--6 players is probably the most comfortable group size.
That said, if you have a huge crew of players, it's probably still pretty balanced. 9 players *can* defeat a 9 hits ghost in a single volley, but that many players trying to take pictures at once is going to be a little chaotic, and might cause some of them to miss. Likewise, 9 players will pretty much always take down a 1 hit ghost in a single round, but with the way the rules are written some of them may still miss and take damage in the process.
If you want to put the pressure on a 9 player group, have them fight three 1 hit ghosts back to back, rather than one 3 hit ghost.