We always have a bag of books checked out from the library. This is our current stash. 10 for AJ, 2 for me.
My rule for AJ is to always have 10 books checked out. Even if she doesn't want 10 while we are at the library (though that rarely happens), I grab a couple extras until we get to the magic number. That way, I always know how many I need to round-up on the due date.