To find out, SF writer Tobias S. Buckell ran an entirely unscientific but still illuminating poll at his blog, and has posted the results.
Here’s the core:
32% wrote one novel
13% wrote two
11% wrote 3
8% wrote 4
9% wrote 5
3% wrote 6
13% wrote 7 or more novels
6% wrote some short fiction first
5% wrote a ton of short fiction first
I took part, and although I think the first book I sold was the fifth one I’d written (that’s what I put on the survey), that was the fifth one I’d written out of high school–throw in the three novels I wrote in high school and Soulworm, my first, actually put me in that “7 or more novels” category that 13 percent fell into.
An interesting survey, which you can still take (if you’re a published novelist) here.