Probably 8 published for every unpublished one. Although I also have a few pages intended for publication which never made it and some prelim's I don't count.
To answer this question:
There are three reasons for me. First, there are artists who I specifically want to draw a Phantom Stranger image which otherwise would not exist. Second, I can't find one for sale, but I can find the artist. Third, I want a specific scene or variation which isn't in the market, like two characters sharing a beer at a bar, or Amy Reeder sketching my girlfriend, that sort of thing.
But no, I would not ask the regular artist on a Batman book, for example, to draw a Batman.