Numbers 4:42
And those that were numbered of the families of the sons of Merari, throughout their families, by the house of their fathers, And those that were numbered of the families of the sons of Merari, throughout their families, by the house of their fathers,
The numbering counted the families descended from Merari, organized by their families and by their fathers' houses, as part of the Levites set apart and enrolled for service. That service was an honored trust centered on the tabernacle, requiring them to erect it and encamp around it, and then to take it down and carry it with its sacred furnishings whenever Israel journeyed. The Levites received this trust because they had remained faithful at the time the golden calf was made and had carried out God's command against the idolaters.