Numbers 4:32-36

and the pillars around the courtyard and their bases, their pegs, and their ropes, with all their equipment and with all their service; and you shall assign by names of the men the items that each is to carry.

This is the service of the families of the sons of Merari, according to all their service in the tent of meeting, under the direction of Ithamar the son of Aaron the priest.”

So Moses, Aaron, and the leaders of the congregation counted the sons of the Kohathites by their families and by their fathers’ households,

from thirty years old and upward even to fifty years old, everyone who could enter the service for work in the tent of meeting.

Their numbered men by their families were 2,750.