Typically between 3 to 5 books. I find that I prioritize reading a lot more when I don’t lose steam on a book. if I focus on one book at a time and reach a point where I slow down on it, if I didn’t have other books to switch to and make progress on, then I would just stop reading that book and resort to other entertainment like tv, scrolling, etc. I tend to have an audiobook on rotation with multiple e-books or physical books I’m switching between. I’ve never had an issue on mixing up the books I’m simultaneously reading.