Joshua 21:40

‘ All these were the cities for the children of Merari according to their families, who were those remaining of the families of the Levites. And their lot was twelve cities.'

So forty eight cities were set apart as levitical cities, that is were put under the authority of the Levites who were given dwelling and pasturage rights. There were thirteen for the family of Aaron (who would in number be few at that time), ten for the remainder of the Kohathites, thirteen for the Gershonites and twelve for the Merarites.