Read Numbers 4:37 Using Other Translations
These were they that were numbered of the families of the Kohathites, all that might do service in the tabernacle of the congregation, which Moses and Aaron did number according to the commandment of the LORD by the hand of Moses.
This was the list of the clans of the Kohathites, all who served in the tent of meeting, whom Moses and Aaron listed according to the commandment of the LORD by Moses.
So this was the total of all those from the Kohathite clans who were eligible to serve at the Tabernacle. Moses and Aaron listed them, just as the LORD had commanded through Moses.