Numbers 4:29

As for the sons of Merari, thou shalt number them after their families, by the house of their fathers;

The instruction belongs to the numbering of the Levites, whom God set apart to serve at the tabernacle in erecting and taking it down and in carrying and caring for its furnishings. It assigns particular Levite families their duties in carrying and caring for the tabernacle, showing an ordered and specific arrangement for the sacred work. The sons of Merari were to be counted by their families and by the houses of their fathers, placing them within the Levite clans under that ordered arrangement.

Their distinct share of the work was to care for the pillars, sockets, boards, and related parts of the tabernacle. Their tents were set on the north side of the tabernacle. Sacred duties were thus specifically assigned and organized by God, not left to individual choice.