1 Samuel 17:40: He (David) took his stick in his hand and chose for himself five smooth stones from the brook, and put them in the shepherd’s bag which he had, even in his pouch, and his sling was in his hand; and he approached the Philistine.
The Bible does not specifically say why David chose five stones. It is likely he was simply being prepared. For example, Matthew Poole’s Bible Commentary says: “…that if one should fail him, he might make use of another.” If this is the case, David took a number of stones so that each could be used in turn, if necessary, as one could not be sure ahead of time whether the first shot would hit.
Quote from the publication Israel Today: “David understood that in order to achieve full victory, Israel must cast out all the giants from the land. You cannot deal with just one stronghold (or sin), but must be prepared to go all the way. Better not to cast that first stone only to discover we are in a battle for which we are not prepared.”
Others surmise David was prepared to fight Goliath’s four brothers if necessary, so he chose one stone per brother. However, did young David – who was not a warrior before his encounter with Goliath and likely not previously aware of Goliath – know the specifics about Goliath’s family members? The incident where David was almost killed by a giant named Ishbibenob in 2 Samuel 21:15-22 was years later.
However, there may be a symbolic reason why the number of stones was five, and that the number was expressed to us in the record. (For it might have just said he picked up a few stones, without revealing the number.) Perhaps David’s victory over Goliath is symbolic of a victory of Jesus over the large statue of four world empires in Daniel chapter 2 — gold, silver, brass, iron. The same is referred to in Daniel 7 as four beasts representing four empires — lion, bear, leopard, dreadful beast with iron teeth — Babylon, Persia, Greece, Rome.
When David was preparing to encounter Goliath, he mentioned that God had delivered him before from the lion and bear (first two kingdoms symbolically), and Goliath is described as having weapons and shields of brass and iron (last two kingdoms symbolically) — so that all four kingdoms are referred to. (1 Samuel 17:34, “Thy servant kept his father’s sheep, and there came a lion, and a bear, and took a lamb out of the flock.”) (1 Samuel 17:6,7 — “He had greaves of brass upon his legs, and a target of brass between his shoulders and his spear’s head weighed six hundred shekels of iron.”)
In Daniel 2, a single stone was used, cut out without hands — representing the true church selected from this world, cut out by the holy Spirit, in order to overcome the kingdoms of this world.