The marketplace products count users the same way our on-prem products do. Specifically, for this offering you can have up to 20 named users that either publish or view on RStudio Connect. We don't differentiate, because we believe there should be no constraints for publishers to view, and viewers to publish.
In addition to the 20 named users, Connect allows for anonymous viewers. You can have up to 20 anonymous viewers concurrently, but there's really no limit on the number of anonymous viewers who could view your app over time.