Do libraries have enough shelf space for all of their books at once?
No. They have offsite stacks. Where books go so you’ll never find them again (unless they were really, really well catalogued; and even then, it’s not the same as browsing shelves).
University libraries have peaks and troughs of demand, and in the trough, there will be enough room for books on the shelf…
… and if there isn’t, that’s when more books are sent to the stacks.