Numbers 31:5 (BSB)

So a thousand men were recruited from each tribe of Israel—twelve thousand armed for war.

From Numbers 31. Also in the ESV.

Commentary on Numbers 31:5

• John Gill (Reformed Baptist), Exposition of the Old and New Testaments on Numbers 31:5: So they were delivered out of the thousands of Israel,.... Several thousands very probably offered themselves voluntarily to go to the war, or however were summoned together on this account, and out of them the following number was delivered of picked and chosen men: a thousand of every tribe, twelve thousand armed for war; which was but a small number to go against so considerable...
• Jamieson-Fausset-Brown (Reformed), Commentary Critical and Explanatory on the Whole Bible on Numbers 31:5: there were delivered--that is, drafted, chosen, an equal amount from each tribe, to prevent the outbreak of mutual jealousy or strife. Considering the numerical force of the enemy, this was a small quota to furnish. But the design was to exercise their faith and animate them to the approaching invasion of Canaan.
• Keil & Delitzsch (Lutheran), Biblical Commentary on the Old Testament on Numbers 31:3-6: Num 31:3-6 To carry out this revenge, Moses had 1000 men of each tribe delivered (ימּסרוּ, see at Num 31:16) from the families (alaphim, see Num 1:16) of the tribes, and equipped for war; and these he sent to the army (into the war) along with Phinehas the son of Eleazar the high priest, who carried the holy vessels, viz., the alarm-trumpets, in his hand.