Read Numbers 4:45 Using Other Translations
These be those that were numbered of the families of the sons of Merari, whom Moses and Aaron numbered according to the word of the LORD by the hand of Moses.
This was the list of the clans of the sons of Merari, whom Moses and Aaron listed according to the commandment of the LORD by Moses.
So this was the total of all those from the Merarite clans who were eligible for service. Moses and Aaron listed them, just as the LORD had commanded through Moses.