I don't think the amount of yeast has much to do with the level of carbonation. That's the sugar's job. You could put an entire yeast starter in it at bottling and it would only eat the amount of sugar that's in the liquid. As Denny says, a portion of a package of dry yeast is probably sufficient if you think there's no yeast in suspension. If the beer lacks Oxygen, dry yeast will supply enough of that as well.