I voted 4-5, as it is my modal category. But at any one time I might have only a couple on the go or 7-8. I am a reading addict and will frantically read train timetables or government advice or anything in print if I don't have what I really want. I typically stock the loo with reading material and read while cooking or cleaning my teeth. I often have a book that stays in the kitchen to be read while I am cooking. A NF book that is a series of short articles is ideal I read The Ancestor's Tale by Richard Dawkins that way. A tooth-cleaning book that I recently finished was Alan Sokal's Beyond the Hoax.
And if I am tired, it's no good my engaging with a challenging book, so I retreat to escapist fiction late at night to get myself to sleep.
BUt I also read a lot of magazines. I am gradually reducing my subscriptions but retain New Scientist (weekly) and am still getting Time. I was in England last week and I came back with a Spectator, Private Eye, and The Economist. I am still reading the last and two New Scientists await me as well as a Time. I also keep up with various newspapers online. As a result my reading levels are very uneven.