How many of the Levites' cities were to be cities of refuge?

Answer: Six

Six of the Levites' 48 cities were cities of refuge, where someone who killed a person unintentionally could flee.

“Six of the cities you give the Levites are to be appointed as cities of refuge, to which a manslayer may flee. In addition to these, give the Levites forty-two other cities.”— Numbers 35:6 (BSB)