Usually one at a time, sometimes more. When I go to the library and check out around a dozen or more at a time I read them all in a row, sometimes skipped around a bit, but mostly one at a time.
When I was in school I would read my books inside the classroom books, got in trouble for it a lot
But couldn't stop reading. Then in college I would read my book of choice plus whatever I had to for the class, but then again, those are special circumstances.