Parashas Bamidbar · Passuk 3:32
The head chieftain of the Levites was Eleazar son of Aaron the priest, in charge of those attending to the duties of the sanctuary.
Chabad.org — English translation + Rashi
Source: Arachin 11b.
English: “The chief of the chiefs of Levi, Elazar” — R’ Yehoshua ben Levi said: Elazar was the officer of the officers.
In context: He held authority over the other Levitical chiefs; the doubled language of “nasi nesi’ei” marks superior rank.
Custom
Chazal describe Elazar as the head of all the Levitical chiefs.