You may kick Temporal Firestorm only once for its {1}{W} cost and only once for its {1}{U} cost. You can kick it twice by paying {2}{W}{U}, but you can't kick it twice by paying {2}{W}{W} or {2}{U}{U}. You can't kick it more than twice. Notably, this means that X will always be either one or two if Temporal Firestorm was kicked.