I'm probably middle of the road. I have about 60-70 books on the kindle - mix of fiction and non- fiction so all my moods are covered. I usually move a series off once I finish it, but keep some old favourites all the time, in case I'm get stranded on an desert island.
I haven't kept a large number loaded, as I use the dictionary a lot, and don't want to slow the reader down with too many books. However, I've noticed it's much faster looking up words on my new kindle touch - so I may load a few more.
Mostly, I don't want to annoy Amazon, with too many books bought on discount from kobo, and transferred to the kindle. My way of keeping my head down