If you want 1 mafia member for 5 - 7 players, 2 for 8 - 10 players, and 3 for 11 - 15 players. I have a suggestion for you ^^ (I do not know if this is the best or optimal way to acheive that, but hope this suggestion helps).
Place a trigger device and make sure it triggers everytime a player spawns. If you need 2 mafia for 8-10 players, go to the option in the trigger called ‘transmit every X trigger’ and put the value to 8.
That means everytime a player spawns on the spawn pad it will send a signal to the trigger, but the trigger will only send a signal to the next device when atleast 8 players are spawned because u put the value to 8. So, when 8 players spawned it will send a signal to a new device which will make 2 players the mafia. For 11-15 players put another trigger which will send a signal when 11 players spawned and will also disable the other triggers which activates when 8 or 5 players are spawned. I know this sounds complicated but make sure u read it slowly to understand this. My bad for being so bad at phrasing words but this is the best i could come up with.