How many books can you put into an empty backpack?

“Zero”

This implies that the backpack cannot hold any books, which contradicts the riddle's premise of it being empty.

“Infinite”

While it might seem like a backpack can hold many books, the riddle specifically highlights the state of the backpack after putting in one book.

“Just one until it's full”

This answer misses the key point of the riddle, which is that once you put in one book, the backpack is no longer considered empty.