Charles Spurgeon Commentary Numbers 4:1-2

"And Jehovah spake unto Moses and unto Aaron, saying, Take the sum of the sons of Kohath from among the sons of Levi, by their families, by their fathers` houses," — Numbers 4:1-2 (ASV)

There were three families, those of Kohath, Gershon, and Merari, and to each of these families a different service was allotted.

1. They were to be numbered. The Lord knoweth them that are his, and he takes count of all his people.