He took his staff in his hand, and chose him five smooth stones out of the brook, and put them in the shepherd's bag which he had, even in his wallet; and his sling was in his hand: and he drew near to the Philistine.
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.