A report by the Transport Committee of the European Parliament was highly critical of Greek national and local governments for failing to control visitation to Santorini, the Ionian Islands and the Peloponnese, to the extent that the attractiveness of these destinations is threatened by overtourism.
Santorini alone received 5.5M tourist visits in 2017, including up to 18,000 cruise passengers per day.
FYI, just to put this in perspective: the island of Santorini has a land area of less than 30 square miles - only about 5/8 the size of the land area of the City of Boston.