In the rare case where enough -1/-1 counters are put on Aerith Gainsborough at the same time to make its toughness 0 or less, the number of +1/+1 counters on it before it got those -1/-1 counters will be used to determine the value of X in Aerith Gainsborough's third ability. For example, if there are two +1/+1 counters on Aerith Gainsborough and it gets three -1/-1 counters, you'll put two +1/+1 counters on each legendary creature you control when Aerith Gainsborough's third ability resolves.