Pressbooks does not support automatic creation and numbering of end-of-book endnotes. If you do not need your endnotes to link back and forth (ideal for ebooks, but not necessary), then you can create your endnotes as follows:
If you need your endnotes to link back and forth then you need to create internal links, then do as above, with the following additions:
1. Create your endnote marker, linking to your endnote page
2. Create your endnote page, with notes linking back to the note inside your chapters
For more details on linking internally, see: Adding Hyperlinks, Internal and External.