David said, Yahweh who delivered me out of the paw of the lion, and out of the paw of the bear, he will deliver me out of the hand of this Philistine. Saul said to David, Go, and Yahweh shall be with you.
Some students of this passage have suggested that David chose five stones because Goliath had four brothers, and he wanted to be ready to attack them too. However there is no indication in the text that David had any concern for them or even that they were present at this battle. He probably chose five stones simply so he would have some in reserve if his first shot missed its mark.