Numbers 31:1-5

Then the Lord spoke to Moses, saying,

“Take vengeance on the Midianites for the sons of Israel; afterward you will be gathered to your people.”

So Moses spoke to the people, saying, “Arm men from among you for the war, so that they may go against Midian to execute the Lord’s vengeance on Midian.

You shall send a thousand from each tribe of all the tribes of Israel to the war.”

So there were selected from the thousands of Israel, a thousand from each tribe, twelve thousand armed for war.