I usually read about 4-5 at a time, mix of fiction and nonfiction, ebook and audiobook, cycling through each. To me it is not much different than juggling TV series, where there is delay between each episode. Or like school where there are different reading assignments for different subjects. There's time to discuss, reflect, integrate what I'm reading, whereas reading something straight through may not be as memorable or enjoyable. Not that I'd know much about reading like that.
My latest experiment involves re-reading a book that I haven't yet finished reading (it is over 1000 pages long). So I was 40% of the way through and then started listening to the audiobook version from the beginning, as I continue reading further in the book. Of course it will take me longer to finish this way, and it is not going to become my standard practice, but I think I'm getting more out of it, at least with this particular book.