And Eleazar the son of Aaron the priest shall be chief over the chief of the Levites, and have the oversight of them that keep the charge of the sanctuary.
The Levites being granted to Aaron to minister to him, they are here delivered to him by tale, that he might know what he had, and employ them accordingly. Observe, I. By what rule they were numbered: Every male from a month old and upward, Num 3:15.
Commenting on Numbers 3:14-39
Of Merari were the family of the Mahlites, and the family of the Mushites,.... So called from his two sons Mahli and Mushi, Num 3:20, these are the families of Merari; the youngest son of Levi, Num 3:17.
chief--rather, "chiefs" of the Levites. Three persons are mentioned as chiefs of these respective divisions [Num 3:24, Num 3:30, Num 3:35]. And Eleazar presided over them; whence he is called "the second priest" (Kg2 25:18); and in the case of the high priest's absence from illness or other necessary occasions, he performed the duties (Kg1 4:4).