Numbers 4:30
From thirty years old and upward even unto fifty years old shalt thou number them, every one that entereth into the service, to do the work of the tabernacle of the congregation. From thirty years old and upward even unto fifty years old shalt thou number them, every one that entereth into the service, to do the work of the tabernacle of the congregation.
The numbering counted Levites from thirty years old and upward even to fifty years old to enter into service to do the work of the tabernacle of the congregation. That work was the practical care of the sanctuary in its journeyings, erecting it, taking it down, and carrying it with its sacred furnishings. The service was not given in bulk but divided among the descendants of Levi's three sons, each clan with its appointed charge.
The Kohathites cared for the ark and the other furniture, the Merarites for the structural parts, and the Gershonites for the curtains and hangings. This carrying, setting up, and taking down of the sanctuary was carefully ordered and entrusted to designated, appointed workers, not open to any tribe or person who chose to take it up. Each worker was to perform the duty assigned to him in his particular place and responsibility, and to leave alone the work entrusted to another. It was assigned service for the tabernacle, not a claim to priestly or governing authority.