David knew his answer was not in his catapult. If he had been certain of his ability to use it, he would have taken only one stone instead of five. The young shepherd had no illusions regarding his own power. He was completely reliant on the Almighty.
Bill Wilson, Whose Child is This?
Others speculate that David chose five smooth stones because Goliath had four brothers, and David was readying himself to dispatch all five giants. This theory is based on 2 Samuel 21:15–22. That passage lists four very large Philistines who were related to Goliath in some way: Ishbi-benob, Saph, Goliath, and an unnamed giant with six fingers on each hand and six toes on each foot. “These four were born to the giant in Gath, and they fell by the hand of David and by the hand of his servants” (2 Samuel 21:22). The account of 1 Samuel 17 does not suggest that David knew about Goliath’s brothers or that he believed he would have to fight the whole family. What we know for sure is that David and his men faced four other giants, somehow related to Goliath, in later battles.
David’s faith was in the Lord, and he knew from experience God’s faithfulness. David’s faith was born out of his experience of God’s grace and mercy in his life up to that point. The Lord had delivered him out of dangerous situations in the past, proving His power and trustworthiness, and David relied on Him to deliver him from the Philistine. Whether it took one stone or five, David recognized that the power was not in his sling but in the Lord of hosts. As David wrote later in Psalm 21:13, “Be exalted, O LORD, in your strength; we will sing and praise your might.”