There are seven character classes: Amazon, Assassin, Necromancer, Barbarian, Paladin, Sorceress, Druid?

If you got to this screen, and you are a nongaming computerphob,
congratulations, that alone is an accomplishment.

Each class has special abilities and special deficits. No character is best for all circumstances. In practice it takes a team with well-built combination in character classes to do the best.

We begin Bardo Training with the barbarian.
However, by the end of your Bardo Training you should have some experience with each character class.

How the characters are used, and the enabling factors, mentations, and inner goals associated with each class is left for intermediate and advanced training.

There is no restriction on the number of different character classes which can be in a game. You can have a game with one of each, or you can have a game that is filled with eight amazons. There is no restriction -- other than the Diablo restriction of only eight characters max in a game.