Romans 4:17 quotes God's promise to Abraham: 'I have made you a father of many ___.'

Answer: Nations

Abraham is the father of all who believe; as it is written, "I have made you a father of many nations."

“As it is written: “I have made you a father of many nations.” He is our father in the presence of God, in whom he believed, the God who gives life to the dead and calls into being what does not yet exist.”— Romans 4:17 (BSB)