The NYPL is a big library system! Currently it seems they have at least 166 copies of Harry Potter & The Sorcerer's Stone[0]. Probably more: I think these are only the copies of the standard edition, and they also seem to have a few copies of an illustrated edition, some special editions, etc., plus of course translations.
Exactly. Remember the days of Blockbuster, they'd have an entire shelf dedicated to 50 copies of a single movie.
Partly to handle actual demand, but, perhaps unlike the library, I imagine it was also partly to generate artificial demand by signaling: "Hey, you, undecided wanderer, look here, this is an important movie that everyone else is renting, you should too."
I remember reading/hearing that blockbuster did that as their competitive advantage over other video stores- they’d build customer loyalty by definitely having the latest thing in stock, even if it was a loss leader.
The first run of the first edition of any book is a prized edition for collectors, but quite a lot of first editions actually go into library systems.
Harry Potter and the Philosopher's Stone (the UK title of Sorcerer's Stone), had a first run of 500 copies. 300 of those went into libraries where they of course were not kept in pristine condition. As a result a fine edition goes for a fair few quid - tens of thousands of dollars, and potentially more.
So, public libraries often have lots of copies of popular books, and may even have several first editions in their collection. They won't be worth much in their usual library state, so anybody thinking of reading this and hoping to steal them: please don't.
Harry Potter was and is super popular. My dinky county library system has 11 physical copies in US english, one in UK english, several non-english languages, and ebooks; our 2010 census county population was only 250k. It's easy to imagine a library that serves a much larger population would need many more copies today, and many more when the book was new, and when the movie was new, and extra copies of the first book when each book and movie came out. Several of my friends would try to read all the existing books timed to finish when a new book was released. Some people would certainly be borrowing those books from the library, especially where people don't have a lot of space for personal libraries.
[0]: https://catalog.nypl.org/search~S1?/Xharry+potter+and+the+so...,