I would think that one (number of books) forces the other (shelf space). My books number over 1200 and even with an eleven foot, floor-to-ceiling shelving area as well as four other free-standing book shelves, I still find myself tripping over piles of books.

I’d call it a library if it is a room for books. I had lots of books, and even lots of shelves, but I didn’t have a library until they came together in one room, with a place to sit and read them.