Ninety-nine souls. And no, they don't have to be a Deathscythe to defeat a Witch. However, a new Weapon with little experience combined with a Meister with a similar status wouldn't be able to defeat a Witch. With more experience, a pair can defeat a Witch; however, it's suggested that you do not hunt any Witches until your Weapon has collected ninety-nine Kishin egg souls.