Thank you for your question Michelle!
You have already found the most relevant guidelines in GSTG.
Beyond that, this would normally be an agency decision as most of the time maps are instrumental in showing the student where things occurred and/or happened, or simply where things are in the world as related to instructional content in a book.
When it comes to implementing any agency decisions, even when there are many items to be included, we would key the items and list them in their respective position on the maps themselves. See GSTG 5.7 through 5.9 for more specifics about formatting keys.
Hope this helps!