The optimum number for the game is 7 - any less and the eliminations go too fast 'cos there's 2 kills per full session - the day (voting) and the night (secret killing) phases.
You can't have a stalemate in other words and odd numbers are better for gameplay and also it's better to have more than 1 cop and 1 killer, as again it finishes too quickly and the killer really is too handicapped. Ideally the innocents (no active role players) should outnumber the active roles by at least 1 to make a good fight of it - hence our preferred 7 minimum. Any lower and you'd really need to be cutting the active roles to even it up a little (so for instance with 6 players you could have 2 killers still, 1 cop and the rest innocents and 2 against 4 with equal active and non-active roles
The Elf game pooh mentioned was unusual and failed for several reasons, one being that Dotsie missed voting because the runner wasn't around and, because we only had 6 'for real' players we voted a cop off by accident so Pooh, who was the other cop, was forced to come out and reveal his ID early and there were all kinds of errors, forced and unforced (like Sjoerd being so 'positive' about my not being a Killer - I was!
) So pooh was right, again by 'accident' although he can sound very sure of himself...
and he persuaded the remaining players to vote me off, but then it came out about Dotsie and the game fell to bits at that point, due to some external factors in that I was having a 19th nervous breakdown in Roundworld
and asked pooh for a truce and to play football with me on Xmas Eve, so we abandoned that one too by mutual agreement, although technically it was an Innocents win or looking that way
The more permutations there are the better the strategy and gameplay needed. I think/hope we can get a 7th person so lets give it another day perhaps? Pooh can decide when he gets on after work anyway and perhaps we can try the 2 killer 1 cop formation instead if we're still 6