From the Introduction:
For Magi, Magic is nothing more than the speaking of two (or more) Words of Power in a magical language said to predate the elves. Magi cannot be multi-classed or dual classed.
Magi begin play at 1st level knowing three Words of Power - either Two Actions and One Realm, or Two Realms and One Action. They learn one new Word of Power every three levels (3,6,9,etc).