I do loads of book covers and I added them all up not so long back - it was over 150. I've done more than that now, and since I often do series, I'm guessing it's closer to 170ish.
I can't remember though whether I included the ones which weren't used. Which, as I've said before, does happen. If that's the case, I tend to show the work anyway - it's irrelevant whether it was used, the work was still created! And often I'm quite pleased with it (not always, mind). You can see how much work goes into them - these are still roughs, not even finished artwork.
Here are a few I've done over the last few months. All of these were rejects. One was eventually used on its Singapore edition. All of them I liked! 'Jepp' was the one I was most disappointed by, as I really thought I'd cracked what was quite a tricky subject matter (you'll probably find it on the US Amazon, if you want the synopsis). Sometimes though, a rejected cover can be revisited for a job further down the line, so it's like having an archive of dormant ideas, waiting to be brought back into gainful employment. Or maybe I'll use just a bit of the idea - like an illustration organ donor!