Just another GR refugee. Other than that, I had a stroke in 2004, and read almost anything I can get my hands on, though I have a particular weakness for history, mystery, and historical fiction.
If counting books "read" in early childhood, I believe Dr. Seuss would be highly competitive. If counting by page numbers, I believe the winner is Colleen McCullough (Her Masters of Rome series is only 7 volumes, but none of them is short, and most are quite hefty). But here's my list for adult authors by number of volumes.
1. Agatha Christie. Her mysteries are like potato chips; I can't read just one. I'm pretty sure I've read at least 30, and there are others I'm not sure about.
2. William Shakespeare. 20 plays and counting. Some read in school, some in the last ten years with a retired English professor and Shakespearian (and a total ham, therefore a very good teacher).
3. Georgette Heyer.
4. Robert Heinlein. I liked science fiction in middle and high school (still do), and Heinlein was one of two science fiction authors my local library had actually heard of. (The other being Isaac Asimov.)
5. Dick Francis. When nursing my grandparents through their last illnesses, I read a lot of Dick Francis, Agatha Christie, and Georgette Heyer, whose novels were all easily available at their very-small-town library, and were all a good way to take my mind off other things.
6. Isaac Asimov. (Tie with Dick Francis.)
Per GR database, #7 is a childhood favorite, L. Frank Baum (creator of Oz), so on to:
7. Dorothy L. Sayers. The creator of the great Lord Peter Wimsey.
8. Roger Zelazny. Interesting FSF author, at both short story and novel lengths.
9. Elizabeth George. Once an auto-buy, and then an auto-get-from-library, and now a "I guess I'll read it if I see it lying around somewhere." Though I still love me some Havers.
10. Tie: Ellis Peters/Steven Saylor/Laurie R. King. All authors of historical mystery series.
Yeah, that looks reasonably accurate.