It's just like the riddle, how many 本 can あなた put on an empty shelf XD Need to think outside the box, i 愛 riddles XD Right my work here is done, i shall possibly talk to あなた soon
Once あなた put a book into a empty bag, it is no longer empty. If you're asking how many 本 would fit into an empty bag, that depends on the size of 本 and the bag. :)
I read "the contest is done the answer is ONE BOOK BECAUSE IT WON'T BE EMPTY AFTER THIS" in the コメント and I am going to disagree, あなた can fit 2, 3, 4, または more, just as long as あなた put them all in at the same time.