These were the descendants of Levi by their families – the heads of families as they were registered under their names and counted individually, that is, the workers twenty years old or more who served in the temple of the LORD.
NIV
These were the Levites by their fathers' houses, the heads of the fathers' houses of those registered, according to the number of names of the individuals who were the servants of the house of the Lord, from twenty years old and upward.
AMP
These were the sons of Levi after the house of their fathers; even the chief of the fathers, as they were counted by number of names by their polls, that did the work for the service of the house of the LORD, from the age of twenty years and upward.
KJV