While researching something else I stumbled over Wikipedia's
List of best-selling fiction authors and thought it interesting. (Note that a number of important authors don't appear on the list because reliable details have not been found - including Dickens and Austen and others.)
First off, imagine publishing 4000 books! That's what Corín Tellado achieved. I think I've left my run a little late. The front runners (of this best-selling list) by volume include:
John Creasey (English, Crime thriller) 600
Barbara Cartland (English, Romance) 723
Enid Blyton (English, Children's literature) 800
Corín Tellado (Spanish, Romance) 4,000
The other end of the scale (by volume) is also interesting. Here are the authors that managed an estimated 100 million sales (or more) with less than 10 books:
Lewis Carroll (English, Absurdist literature) 5
Dan Brown (English,Thriller) 6
Stephenie Meyer (English, Romance) 6
Eiji Yoshikawa (Japanese, Musashi) 7
J. K. Rowling (English, Harry Potter) 11
Arthur Hailey (English, Thriller) 11
(I've included Rowling and Hailey here because I'm pretty sure they were safely above the 100 million sales with less than 10 books.)
I'm not really sure what all this tells us, but I'm still astounded that anyone managed to publish 4000 books.