My business partner and I have a 600+ at present but we will be looking to trim this down after the first of the year due to our changing financial and family obligations.
Currently at around 300 domains: 95% .com, a couple of .Net, Info and Org.
I'll develop a few, learn more about domaining to better understand sell opportunities, and sit on the rest for a while. Have owned most since early 2000's.