reason and experience both forbid national morality to prevail with the exclusion of
5 smooth stones may have been used to kill the linage of the philistine,
the scripture records that Goliath had 4 brothers, thus David may have been expecting to kill Goliath and each his brothers by the grace of God.
However, as i read it the bible doesn't answer this question directly and gives little hints as to why 5 stones, but i am a bit of a mystic when it comes to the scriptures, couple that with an avid Chuck Missler fan and you get rathering inspiring answers to such questions, but what may i ask prompted you to ask this one in the first place? It isn't exactly out of your stock NIV trivia collection.