By “books on human rights” I mean books with “human rights” in the title. The answer is at least 27, the number offered by Amazon when I checked yesterday. Amazon also offers 1,270 books on human rights that were published in 2013; and 18,960 books on human rights in total. (A spot check indicates a small number of false positives but the search is also underinclusive since it is limited to books with “human rights” in the title.)
Amazon does not sell all books. WorldCat, a large collection of library catalogs, lists 6,516 human rights books from 2013, and 166,891 in total.
But there’s always room for one more.