When configuring the maximum number of co-authoring authors as instructed here, what is the recommended maximum? Or is there a guideline for computing the recommended maximum number?
As per the Software boundaries and limits for SharePoint 2013 This property only applies to Microsoft PowerPoint 2010 presentations and Microsoft Word 2010 documents.
Recommended maximum number of concurrent editors is 10. The boundary is 99. If there are 99 co-authors who have a single document opened for concurrent editing, each successive user sees a "File in use" error, and can only open a read-only copy.
More than 10 co-editors will lead to a gradually degraded user experience with more conflicts, and users might have to go through more iterations to successfully upload their changes to the server.
As per the link you mentioned, There is no upper limit to the number of users who can co-author Microsoft OneNote notebooks.