five stones is correct, but as to the why? Hmm... that's a toughie. Off the top of my head: Because not even David was supposed to know that God was going to take care of Goliath with the first stone David slung?
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.