Officially, there are 7 Hunters, (including Samus)
The others include:
(see individual articles for more information)
In Adventure Mode, the player can only control Samus, but in Multiplayer (Multi-Card) and in Wi-fi, you can play as Samus and the other hunters. (In Single-Card/DS Download Play, the host can choose any Hunter, however the person without the game must be Samus). Each Hunter has a different Alternate Form, and also a different Affinity Weapon. The layout of the HUD also differs slightly with each Character.