Change came in the 1950s. In 1958 New York mayor Robert Wagner, Jr. issued an executive order, called "the little Wagner Act," giving city employees certain bargaining rights, and gave their unions with exclusive representation Management complained but the unions had power in city politics. By the 1960s and 1970s public-sector unions expanded rapidly to cover teachers, clerks, firemen, police, prison guards and others. In 1962, President John Kennedy issued Executive Order 10988, upgrading the status of unions of federal workers.

Based on the above article, answer a question. How many specific types of workers are listed as having unions expand to cover them?
5