One of the things I started tracking a couple years ago, out of curiosity, was which authors I'd read six or more books by. I believe this is my current list:
Robert Heinlein (38)
Harry Turtledove (37)
Michael Stackpole (15)
Piers Anthony (14)
C.S. Lewis (11)
Greg Rucka (10)
Michael Crichton (7)
Larry Niven (7)
J.K. Rowling (7)
J.R.R. Tolkein (7)
Margaret Weis and Tracy Hickman (7)
Isaac Asimov (6)
Robert Charrette (6)
Jack McKinney (6)
Michael Moorcock (6)
Unless he turns his writing around, Turtledove won't surpass Heinlein, as I'm quite disenchanted with Turtledove's writing of late. That said, if he does go back to producing tightly written, interesting work, he's a shoo-in for the lead spot, since Heinlein isn't writing any more books. Greg Rucka has been pulling his way up the list with a series of enjoyable books (Patriot Acts, being the latest). Of late, there haven't been many authors for whom I've tried to read my way through their entire oeuvre, so it'll be a while yet before anyone cracks the Heinlein-Turtledove mark.