Numbers 31:32
And the booty, being the rest of the prey which the men of war had caught, was six hundred thousand and seventy thousand and five thousand sheep, And the booty, being the rest of the prey which the men of war had caught, was six hundred thousand and seventy thousand and five thousand sheep,
The 675,000 sheep were counted among the rest of the prey remaining after Israel's campaign against Midian. That campaign was Israel's first taking of captives, when Israel marched into Midian, slew all the males, and seized the women, children, flocks and herds. After the battle Moses challenged and reprimanded the army officers for disobeying God's command by sparing the women and children. All but the virgins and young children were then put to death.
In the division of the Midianite spoil, soul could apply to either a human being or an animal. One out of every five hundred persons and animals taken was set apart. The women were ordered to be killed not only because they were corrupt but because they intended and were able to corrupt others. Preserving those wholly given over to evil would have brought further corruption.
The virgins and children who were spared were reserved for adoption into the families of the nation rather than for enslavement. God's law prohibited the enslavement of prisoners of war, the most productive means of making slaves. By that prohibition Israel was thereby prohibited from becoming a slaveholding nation. The judgments executed on Midian and other nations stand as examples of the judgments to come upon peoples who utterly reject God.