How many sons of Haman were killed and then hanged on the gallows?

Answer: Ten

The Jews in Susa killed Haman's ten sons, and at Esther's request their bodies were hanged on the gallows.

“They killed these ten sons of Haman son of Hammedatha, the enemy of the Jews, but they did not lay a hand on the plunder.”— Esther 9:10 (BSB)