The number of books published annually is somewhere between one million and ten million, depending on how they're counted. Self-published? ebooks? Audiobooks? Etc.

I'm aware:

https://hackertimes.com/item?id=31604033

Keep in mind that a largish municipal library is on the order of 250-500k items.

The Library of Congress includes ~40 million catalogued books, of which ~360k circulate annually.

Again, the question is what Amazon offers.