I think that after 11 or 12 (and there was the spin off set of books with Roxanne and Tempus and Niko) I stopped looking for them in book stores since I moved overseas. I'd need to do some research and get back to you. Somewhere in the attic I have boxes and boxes of books that I refuse to throw away, but I think that I lost at least one box of books during a move.
This may take some doing.