I've yet to hit my limit. Granted, I usually keep my "rotation" to three or four at a time, just because I tend to get waaaay too wrapped up in the actual reading, and will spend every ounce of my time doing it.
No joke, I've been known to fall asleep on the toilet with a book in my hand. Not because I was constipated, or the book was boring. I would "forget" where I was and just read until I passed out.