Around 4. When I read 1 book at a time, I find I read less often and for shorter, and I tend to get annoyed if the book is slow or not interesting. If I can pick what I want to read more it feels easier to sit down and read something. I'm not reading 4 fiction books of the same genre though. Typically, it will be 1 non-fiction and 2 novels of different genres, and 1 short story collection that I barely touch.