The number of changes/edits is what is considered, wit openstreetmap-website/app/controllers/api_controller.rb at master · openstreetmap/openstreetmap-website · GitHub
It should be noted that the relationship between changes made by an user in an editor and what gets uploaded and recorded as a change isn’t directly one to one or as simple as you think it may be (but that is likely not an issue in the context of Missing Maps).