Why did David need 5 stones to fight Goliath?

David's Sling
Sling and five smooth stones used by David to kill Goliath on a white background

Everyone has heard about the battle between David and Goliath, how David was just a boy, and Goliath was a giant. The best guess of Goliath’s height was 9ft 9inches. (converting cubits to feet is no certain thing.)
David’s chosen weapon was a sling with 5 stones. Have you ever wondered why David decided that he needed 5 stones? Did he think that God was unable to do it with one?
No, the answer is that Goliath had at least four brothers.

In 2 Samuel 21:18-22  It talks about 4 who were killed who were sons of the Giant in Gath. (Goliath’s home) and specifically mentions that one of them was the brother of Goliath.

It stands to reason that if you plan on picking on, or killing someone who has brothers nearby, that you should plan to defend yourself against them as well.

My most commonly used Bible is the 
Apologetics Study Bible
I highly recommend it!