Kurt Vonnegut - 14 novels and around 10 short story/essay collections
Italo Calvino - around a dozen books, both novels and story collections
Isaac Asimov - I think 17 novels and maybe a dozen short story collections
Connie Willis - 5 novels, 3 collections of stories/novellas
Stephen King - 16-17 novels, 6 collections (most when I was a kid)
James Thurber - I dunno, around 9 or 10