Let's see, it looks like 19 for me. (I've stayed over 60 trips now, so most of the resorts have been multiple times.)
Pop Century
A/S Sports
A/S Music
A/S Movies
POR-Riverside and Dixie Landing
PO-French Quarters
Carrib. Beach
Corondo Springs
Wilderness Lodge
Wilderness Lodge Villas
Old Key West
Animal Kingdom Lodge
Animal Kingdom Villas (Jambo house)(Kidani in September this year)
Boardwalk Inn
Boardwalk Villa
Beach Club Resort
Beach Club Villa
Contemporary Resort
Bay Lake Towers
Also stayed at Grosvnor in 1987 before Carribean Beach Resort openned the next year.
I've not stayed at SSR, (nor the Tree House), Camp Ground, Poly, Yacht Club, and GF