The Belgariad:
* 5 books: *Pawn of Prophecy*, *Queen of Sorcery*, *Magician's Gambit*, *Castle of Wizardry*, *Enchanters' End Game*
The Malloreon:
* 5 books: *Guardians of the West*, *The Seeress of Kell*, *The Sorceress of Darshiva*, *The Demon King*, *The Redemption of Althalus*
The Elenium:
* 3 books: *The Diamond Throne*, *The Ruby Knight*, *The Sapphire Rose*
The Tamuli series:
* 3 books: *The Rivan Codex*, *The King's Shadow*, *The Seer's War*
Other series:
* 2 books: *The Redemption of Althalus* (originally a stand-alone book, later considered part of The Malloreon) and *The Legacy of the Master*
Total books: 18 books
So, depending on which series you're interested in, the number of books ranges from 3 to 5.