Here is one list:
https://answers.yahoo.com/question/i...9035815AAZSU8p
Here is another:
http://en.wikipedia.org/wiki/List_of_prolific_writers
Those add Isaac Asimov, Enid Blyton, Alexandre Dumas, and R.L. Stine that I am familiar with and a bunch more that I am not.
Off the top of my head here are some authors I think might meet your 50 books criteria in addition to those ones: Anne McCaffrey (Pern series alone is 30+ isn't it?), James Patterson, Terry Pratchett (Over 40 in the Discworld series).
Edit: Another list:
http://io9.com/11-most-prolific-scie...-of-1443957263