Getting the right number of players for a game is a bit like getting the right amount of spice in a soup. It's partly a matter of personal preference, and partly a matter of what soup you're making.

The traditional number of players in role playing games is five or six.

This stems from the fact that Dungeons and Dragons is what most people started out playing, and a well rounded group of characters would represent the major classes, with a bit of extra combat power. So you'd have a thief, a wizard, a cleric, a couple of fighters, and maybe one of the other classes, too. You needed all those in the game because typical adventures required all the skills from those classes to succeed.

This runs against literary tradition, where you normally have lone heroes going against great odds, or a pair of protagonists fighting the good fight.

So what's right for your game?

Well, if you're running the traditional D&D game, you'll probably not have less than four players, just to make sure you have enough critical skills in play. If you have fewer than that, you'll probably be playing a bit loose with the rules and allowing plenty of creative solutions to problems. For example, if they don't have a thief, allow the fighter to disarm a trap by tossing a sword into the triggering mechanism.

One technique I used fairly often in my D&D campaigns was to run individual adventures for characters. This would be what their characters were doing between group adventures. These would let the thief plan out city jobs, tweak the thieves' guild, etc. A cleric might need to perform a task for her church, or convert unbelievers, or whatever. This allowed each character to shine in their solo adventure, and feel less like a cog in the party.

When I run Call of Cthulhu, I like to have a maximum of four players. While I've run with more, I find it easier to keep everyone involved with fewer. The mood of the game is such that you really want everyone into the game, not waiting for their chance to do something. Three players is probably my minimum with Call of Cthulhu, because you need to allow for some degree of character death or insanity.

Superhero games are another genre where the literary tradition has solo heroes against villains, with the occasional team up between heroes. The technique I used with my D&D campaigns works well with these sorts of games, too. Another that works well is to have each player create a hero and a villain character, and for one player's hero to be the spotlight character for an adventure. The other players would play their villain characters that time.

The perfect number of players also depends on your gaming group. I've played with some groups that needed a large number of players to have the social aspect to play. Other groups have worked well with fewer players.

So you'll have to figure out what works best for your group, based on the games you're playing and the sort of players you have.

Ultimately, though, if you have people willing to play, that's probably your ideal number!