How many sons of Ahab did Jehu have killed?

Answer: Seventy

At Jehu's demand, the leaders of Samaria killed all seventy of Ahab's sons and sent their heads in baskets to Jezreel.

“Then Jehu wrote them a second letter and said: “If you are on my side, and if you will obey me, then bring the heads of your master’s sons to me at Jezreel by this time tomorrow.” Now the sons of the king, seventy in all, were being brought up by the leading men of the city.”— 2 Kings 10:6 (BSB)