13 - that's actually more than I thought I had. After getting the complete series, I decided I didn't need any more "here's a bunch of Races, PRC's, feats, spells and rules to destablize your game!" books. I feel for the RPGA, trying to accomodate such a huge matrix of options. Be interesting to see overtime if they have to take steps to deal with certain combos. I'll pick up some more FR regional books in the end though - I like the fluff.
PHB, DMG, MM, Miniatures Handbook
Complete: Warrior, Divine, Arcane, Adventurer
Player's Guide to Faerûn, Underdark, Shining South
Dragonlance Campaign Setting
Eberron Campaign Setting