It's not just about counting every territory. Aruba, Curacao, and Sint Maarten are all considered to be constituent countries of the Kingdom of the Netherlands. Anguilla and Montserrat are just territories of the UK. The Netherlands also has some territories in the Caribbean that are not considered constituent countries (namely Saba, Sint Eustatius, and Bonaire).
Then why are there Sint Maarten, Aruba and Curacao?