For how many pieces of silver did Joseph's brothers sell him?

Answer: 20

When the traders passed by, the brothers pulled Joseph out of the pit and sold him for twenty shekels (KJV: pieces) of silver.

“So when the Midianite traders passed by, his brothers pulled Joseph out of the pit and sold him for twenty shekels of silver to the Ishmaelites, who took him to Egypt.”— Genesis 37:28 (BSB)