Official documentation on CMS reference limits should be on the hosting plans page. A product has only 3 of 5 references from the get go. I understand that products variants and categories are chewing up a reference straight up.
It is pretty clear to me that most stores are going to need the Plus pricing plan to build a solid site architecture. It is a bit of a shame as smaller stores starting out can still have the same bigger CMS architecture needs. First world problem …?