The usual ratio is one mafioso (or equivalent) for every two innocents.
3 players = 1 villain
6 players = 2
9 players = 3
12 players = 4
15 players = 5
18 players = 6
This game started with 20 players (the most I've ever tried to wrangle at once
), actually one shy of the jump to seven -- but, with four special roles (weaponsmaster, healer, alchemist, historian) I went ahead and used seven rebels. It's generally agreed that the more special roles, the more advantage goes to the innocent players, though again, I have no hope of crunching the numbers. Heh.
But there is also a fair element of luck involved. If the rebels are lucky enough to remove the guardian early, it's much more difficult for the innocents to win. If the seer (historian) ends up spying only on villains the first few rounds, that's useful in some ways, but then the seer also has no one safe to plot with.
I've used the same ratios all along, and so far I've had four innocent victories and two where the villains won.