John Gill Commentary Numbers 4:29

"As for the sons of Merari, thou shalt number them by their families, by their fathers` houses;" — Numbers 4:29 (ASV)

As for the sons of Merari
The third and youngest son of Levi:

you shall number them after their families, by the house of their
fathers ;
which is the form of expression used of them all; (See Gill on Numbers 4:2).