Auto-numbering falls under the umbrella term “Placeholder tags”, which include other info like author and project names. You can find a pretty thorough list and explanation of all of these tags by going directly to the Help->Placeholder tags menu.
Oddly, there isn’t any discussion of how to use these tags in the manual that I can find.
Since the manual is written using Scrivener, it turns out to be surprisingly difficult to include a section about placeholder tags that will remain coherent after compiling. That’s why the information is in a separate document on the Help menu.
Nevertheless, the term “autonumbering” is well understood in the wp/typesetting world. Scrivener seems to partially support the concept yet the term is not used in the manual and the information is hidden, again only partially, behind the term, “placeholder text.” That “autonumbering” is a subset of “placeholder text” is not immediately obvious, to say the least.