The amount of books I read at a time really depends on what book I am reading, and what other books I have. Usually, I stock up on books and I have somewhere around ten to twelve books all at once. If the book I choose to read first is really, really interesting than I'll finish reading it before starting another one. However, if the book is an "okay" type, or slow and kind of boring, than I am going to start reading another book, and hopefully finish the one that I had first started reading sometime later. But if I have multiple books that I am familiar with and it's part of a series I am reading, like sequels or something, than I will also try to read all of them at the same time, because I am not known for patience when it comes to books, or series I love.
Unfortunately, reading more than one book at the same time isn't the best for me because I want to read all of them at once, which isn't possible, and that just makes me go crazy. So because of that, I try to rush through the book I am currently reading, and at the end I feel like I missed out on half the story, which is completely my fault, but still, I don't like it when that happens, so I try to stick to one book at a time. Not that it always works, but I'd rather read a book completely, than going back and forth between two books or more.