The tribes, or families, of Israel descended from the sons of Jacob, whose name was changed to Israel. This patriarch had 12 sons—Reuben, Simeon, Levi, Judah, Dan, Naphtali, Gad, Asher, Issachar, Zebulun, Joseph, and Benjamin. (Genesis 29:32–30:24; 35:16-18) Eleven of these brothers had tribes named after them, but no tribe was named after Joseph. Instead, two tribes were named after his sons, Ephraim and Manasseh, who received full status as tribal heads. So the number of tribes in Israel amounted to 13. Why, then, does the Bible usually speak of 12 tribes?
Among the Israelites, the men of the tribe of Levi were set apart for service at Jehovah’s tabernacle and later at the temple. Hence, they were exempted from military service. Jehovah told Moses: “Only the tribe of Levi you must not register, and the sum of them you must not take in among the sons of Israel. And you yourself appoint the Levites over the tabernacle of the Testimony and over all its utensils and over everything that belongs to it.”—Numbers 1:49, 50.
The Levites did not receive a territorial allotment in the Promised Land either. Rather, they were assigned 48 cities scattered throughout the territory of Israel.—Numbers 18:20-24; Joshua 21:41.
For these two reasons, the tribe of Levi was not generally included when the tribes were listed. The tribes of Israel were thus usually numbered as 12.—Numbers 1:1-15.