John Gill Commentary Joshua 21:40

"All [these were] the cities of the children of Merari according to their families, even the rest of the families of the Levites; and their lot was twelve cities." — Joshua 21:40 (ASV)

So all the cities for the children of Merari by their
families
Which were distributed by lot to them, according to the number of their families, and were sufficient for them:

which were remaining of the families of the Levites ;
the last division of them, besides those of the Kohathites, Levites, and the Gershonites:

were [by] their lot twelve cities ;
four out of the tribe of Zebulun, four out of the tribe of Reuben, and four out of the tribe of Gad, whose names are before given,