The value of X for this card's ability is determined when the ability resolves. When the ability resolves, you will put (10 - X) strength counters on Kirik Rerik, where X is the number of strength counters on Kirik Rerik when the ability first started resolving.
Kirik's automatic ability checks the "X" value on resolution. For example, if you Judgment with 10 strength counters and use all 10 for his activate ability in response to his automatic ability, he will gain 15 counters when his automatic ability resolves.