There are about 16,000 public libraries in the U.S. (that's counting branches; there are about 9,000 administrative library entities).
I was curious about that because the fear seems to be that with current media and economic trends, book sales will fall, while public library usage is up. I use the library constantly and we probably have 4-10 books out at any given time. I'd thought to myself, "Oh, hey, my book doesn't have to be a huge bestseller through stores and Amazon, if it just gets bought by enough public libraries." But alas, even if every library bought a copy of my novel, it still wouldn't earn me enough to pay for, oh, let's say a whole year of my daughter's college tuition. Though it would still be a very good amount of sales for any literary novel, don't get me wrong and I'd be plenty grateful. (And for those libraries who don't already own a copy of Tornado Siren... no time like the present.)