I have somewhere in the 800-1000 range on my kindle, which is basically most of the books that I have that I'd ever consider rereading, and books that I haven't read yet that I do want to read. My wife's kind had about 3200 on it, but we were having some issues with it rebooting a lot, and taking forever to do so, so we've just cleaned it down into the 800 range also.
I like having everything I might want to read on the kindle. To me, that's part of the point in having it.
While I normally do read all the way through a book before moving on to some, sometimes I just get in a mood where I just don't want to finish reading whatever it is I started, and will move on. And sometimes I've started a book that I decided was just so bad, I didn't see the point in finishing it.