Ninja Sword and Pole (Sometimes Axe, Greatsword and Sword) always do multiple hit. I think Vitality have something to do with it. Balthier never really pass 4 hit in my data but he has the best speed. My Ashe have Pole equipped and she likes do more than one hit, same with Penelo (I give her Katana).

Masamune + Genji Gloves = Always do more than one hit.