You have to know for yourself; Does (for example) 4 characters seem too less, too many or just about fine, and what about 3, or 5? Or 10, or 1?
This also differs to how the battle system works, and your own opinion; In a turn-based battle (standard one) for example I find 4 characters fine, but in a real-time battle on the map (like WoW but 2D) I like about 8 characters better; More chaos, more co-operation. (Someone else might find it too much chaos?)
This also differs to your class system, and the entire gameplay.
I don't know about your game, but all the advice I can give is: "Just see what feels right." (Simple but true)
Haven't you ever noticed that "four" is the only number with the same amount of letters as it's definition in the entire English language?