I’ve been to 21 so far. The rules apply to the park the team is currently playing at. If a team moves or a new park is built those will be circled back to after 30 have been completed.
For example….I have been to the Twins, Marlins, Braves, and Rangers ball parks but have all since moved to new stadiums. These will be revisited after I visit each teams current park at the time of my visit.