Jamieson, Fausset and Brown on Numbers 1:4-18

4-16. with you there shall be a man of every tribe, &c.--The social condition of the Israelites in the wilderness bore a close resemblance to that of the nomad tribes of the East in the present day. The head of the tribe was a hereditary dignity, vested in the oldest son or some other to whom the right of primogeniture was transferred, and under whom were other inferior heads, also hereditary, among the different branches of the tribe. The Israelites being divided into twelve tribes, there were twelve chiefs appointed to assist in taking the census of the people.