What does Isaiah 13:12 mean?

Isaiah records God's declaration that He will not allow proud, evil humanity to continue to thrive in the world. The day of the Lord's judgment is coming for them. This judgment is coming due to their nauseating combination of wickedness and arrogance, as well as ruthlessness and vanity. The Lord has said that He will put an end to it (Isaiah 13:11).

In this verse, the Lord adds that the body count will be astronomical. His judgment will reduce the population of the people of Babylon so that human life becomes rare in their land. Living, breathing people will become rarer than fine gold. He compares this scarcity to the "gold of Ophir." In that era, gold from Ophir was famous for purity. In the glory days of Israel, Solomon sent out a fleet of ships that returned from Ophir with 420 talents—many tons—of this pure gold (1 Kings 9:28). Ophir may have been located somewhere in East Africa.

The Lord's point is not meant to be taken as a math formula, but as a warning: after His judgment on Babylon has fallen, it will be easier, metaphorically, to find this precious metal than to find living people.