As many as are needed for the playstyles you want to encourage. How long is a piece of string?Was wondering this when it comes to new editions of Dungeons and Dragons. How many classes are too many and how many are too little?
Is it for flavor purpose and fulfilling certain archetypes? Having certain roles be fulfilled?

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.