The Levites were exempt from military service; hence they were not numbered with the tribal forces. Levi was a tribe devoted especially to the service of God

 (v. 50) in place of the first-born (ch. 3:12). There were 12 tribes besides Levi, because Jacob had adopted Ephraim and Manasseh, the two sons of Joseph (Gen. 48:5, 6); consequently, instead of one tribe of Joseph, there were two separate tribes of Ephraim and Manasseh. A separate command from Jehovah was given in respect to the Levites (Num. 1:48-50). Their duties were to be of a different nature, and are stated more fully in chs. 3 and 4.