Jack's soul = 100
Okay, at first I thought Davy Jones just said "100" just to make the mission of collecting that many souls impossible.
But isn't it just a teeny weeny bit of a coincidence that Jack, during his earlier days, saved NINETY NINE SLAVES. Which then made his soul much, much more valuable, because of his kind act.
so 99 + 1 (his soul) = ONE HUNDRED