Hussar
Legend
Working from the LibraryThing series lists, there have been about three hundred supplements for GURPS. D&D 3.0 & 3.5 each had about 50, AD&D 1 had almost 100, and AD&D 2 had almost 350. This only counting official TSR/WotC stuff.
I think the D&D numbers are very, very low. Heck, there was 350 DRAGON magazines and another 150 (or so) Dungeon mags in print for D&D.
There's a poster around here who's actually done the numbers but, I can't for the life of me remember who it was.