Remathilis
Legend
I would say three.
Warrior = Fighter, Barbarian, Paladin, Monk, Warlord.
Rogue = Ranger, Rogue, Assassin, Bard
Mage = Wizard, Sorcerer, Cleric, Druid, Warlock
Why not 5?
Warrior
Mage
Explorer
Diplomat
Rogue
Don't laugh, /no classes/ is an option that's worked very well for a lot of games.
There are a lot of games that having 2, 3, 4 or no classes at all would make sense.
None of those games are D&D.
You can make all the well-balanced games in the world with 1d4 generic classes and a wealth of backgrounds, kits, themes, prestige classes, paragon paths, and whatever else you want to completely mimic any class of yore, and the FIRST THING people will do is want their Ranger, Bard, and Druid classes back.
Since we're talking about monks, lets review what happened when TSR took them out of the game in 2nd edition. They told people to buy Oriental Adventures (a 1e product). They make a priest kit in 2e. (Perhaps more than one?) They brought it back as a weird spellcasting Priest-class in Faiths and Avatars. Finally, they reprinted it in Scarlet Brotherhood in its full glory after dozens of fan-made conversions and half-baked attempts for the better part of eight years. People wanted their monk. They didn't want lousy kits or Martial Arts proficiencies. Luckily WotC figured that out and put them in the PHB for 3e.
I say get it out of the way. People will WANT Paladins, Rangers, Bards, Warlords, and Monks, and they should be allowed to have them. D&D is famous for them. Leave the generic classes for the Fantasy Heartbreakers and OGL spinoffs. If I'm playing D&D, I want Paladins, Monks and Rangers as full-bodied classes, not Generic Fighting Man (Flavor to Taste).
You can't make a "Unity" edition by removing 9/10ths of the classes people expect to be there...