Which of these was a king of Israel told not to acquire in great numbers?

Answer: Horses

The king must not acquire many horses, take many wives, or accumulate large amounts of silver and gold.

“But the king must not acquire many horses for himself or send the people back to Egypt to acquire more horses, for the LORD has said, ‘You are never to go back that way again.’ He must not take many wives for himself, lest his heart go astray. He must not accumulate for himself large amounts of silver and gold.”— Deuteronomy 17:16-17 (BSB)