John Gill Commentary Numbers 3:20

"And the sons of Merari by their families: Mahli and Mushi. These are the families of the Levites according to their fathers` houses." — Numbers 3:20 (ASV)

And the sons, of Merari, by their families
The third son of Levi; Mahli and Mushi ;
the same as in (Exodus 6:19) ; from whom were denominated the families of the Mahlites and Mushites, who, as the preceding families, were numbered at this time: these [are] the families of the Levites, according to the house of their
fathers ;
in all eight families.