C. F. Keil and Franz Delitzsch on Numbers 4:2-3

Num 4:2-3 Service of the Kohathites, and the number qualified for service. - Num 4:2, Num 4:3. “Take the sum of the sons of Kohath from among the sons of Levi:” i.e., by raising them out of the sum total of the Levites, by numbering them first and specially, viz., the men from 30 to 50 years of age, “every one who comes to the service,” i.e., who has to enter upon service “to do work at the tabernacle.” צבא (Angl. 'host') signifies military service, and is used here with special reference to the service of the Levites as the militia sacra of Jehovah.