I try to read only one at once, but sometimes I pick up a second or third book. Sometimes its because I didn't finish reading a library book before it had to get returned (because I read books mainly from the online library this happens automatically). Sometimes its because I just don't feel like reading my current book either because of recent events, my current mood, or health. At the moment I think my Goodreads is at around 10 books I'm currently reading.