A famine occurred in Israel during the days of David. When he sought the Lord about it, He revealed to the king that King Saul had broken a promise to the Gibeonites, made by Joshua and the elders centuries earlier, to protect the Gibeonites. Instead Saul had killed many of them.

David went to the Gibeonites to find out what they wanted done,

Then they answered the king, “As for the man who consumed us and plotted against us, that we should be destroyed from remaining in any of the territories of Israel, let seven men of his descendants be delivered to us, and we will hang them before the Lord in Gibeah of Saul, whom the Lord chose.” And the king said, “I will give them” (2 Samuel 21.5–6). 

And so David delivered them. Seven men of the descendants of Saul would suffice, showing how people often recognized the number 7 as a complete and perfect number.