When there are several appendices, what is that part of a book called?

Really, there's no standard except whatever the publishers, authors, and editors agree on. I've seen both singular and plural deployed in this way, in different books.

You could call it the Back Matter, which avoids the Singularity, putting it in a class with the Index, Colophon, Cast of Characters, Diagrams, etc.

As an example, Dorland's Illustrated Medical Dictionary (2,000+ pages) has a section at the back labeled "Appendices" with a table of contents listing all of the following appendices. Then each appendix is labeled on its respective page "Appendix 1" through "Appendix 18."

I would call this section of the book the Appendices. (That's also what Dorland's calls it in the table of contents at the front of the book.)