According to the author of the book of Joshua, God gave the families of Merari, who were part of the helpers in the temple, twelve cities to live in. This was part of God's plan to make sure all the helpers had places to stay.
Joshua 21:40: So all the cities for the children of Merari by their families, which were remaining of the families of the Levites, were [by] their lot twelve cities.
This verse enumerates different numbers for the cities with their suburbs given to the Levites, in comparison to those listed in Joshua 21:40.
1 Chronicles 6:66: And [the residue] of the families of the sons of Kohath had cities of their coasts out of the tribe of Ephraim.