What SF author has had the most works adapted for film?
I couldn't give him an accurate answer on short notice, but I dashed off a quick response, and I'll share it with you. Chime in if you can help.
I think Mary Shelley beats everybody on "most films made from her work," but that is not exactly your question. Verne, Wells, Bradbury, and Dick are leading contenders.
John Scalzi thinks Wells is the champ, but offers no statistical evidence.
Also not exactly answering your question, the Internet Movie Database lists both movies and TV shows. For "writer" it says:
Author Number of Titles in IMDB Jules Verne 143 titles Stephen King 127 (mostly horror, some SF, some non-SF) H. G. Wells 85 titles Ray Bradbury 75 titles (some non-SF) Richard Matheson 75 (a ton of TV, but plenty of features) Mary Shelley 57 Robert Bloch 44 (mostly horror, some non-SF) Michael Crichton 28 (several are not SF) Philip K. Dick 21 (very few remakes, unlike some of his seniors) Isaac Asimov 18 Robert Heinlein 12 (5 feature films, the rest TV) Arthur C. Clarke 11 (2 feature films, the rest TV or in-development)
Though he has also enjoyed (suffered?) innumerable adaptations, I won't count Bram Stoker as an SF author. But the others have all written at least some SF.
Can't guarantee this is exhaustive, but on short notice, it may give you a handle on the question. Hope the person you're betting against is still in the bar.
It would take more work to count only adaptations and determine a champion. Has this already been tabulated somewhere?
(Uh-oh: for creators of TV series, IMDB doesn't count every episode as a "title" in its summary for "writer" credit. So these figures may be off for TV writers. J. Michael Straczynski has to be in the running for most prolific SF author on film, as he wrote the vast majority of scripts for the 110 episodes of Babylon 5. But not one of them is an adaptation.
(Furthermore, although there are hundreds of films derived from Frankenstein, IMDB appears to count only those that gave Mary Shelley a "writer" credit. She should talk to her union rep.)