It'll vary depending on the numbers in the game.

If you're doing a traditional mafia, it generally runs about 1 mafioso/SK for each 7-8 entrants. GH's classic typically uses 3 mafiosi, 1 detective, rest town, regardless of numbers. The key is: mafia can coordinate, townies have preponderence of numbers but no real coordination.

My capo's are mafia wars so the numeration is different.