And Eleazar the son of Aaron the priest was to be chief over the chiefs of the Levites, and to have oversight of those who kept guard over 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).