My rule of thumb has been take the minimum number of games and multiply by two. So if it's a 5 game guarantee you need 10 pitchers. then again, this also depends how talented your pitchers are.
My son's 15u team played in a couple 7 day PG events and with 17 rostered kids (12 pitchers in there) we were tight on pitching after the first bracket game.
How you pitch your kids and what day you pitch them has such a huge impact with Pitchsmart rules. If your last pool game is against the other tough team and you save your ace for that game then you won't have him for bracket play. BUT, you won't make bracket without him because only the top team from each pool advances. Double edged sword.