Numbers are for illustration only -- if the next pope decides to skip a number or repeat a number for whatever reason, it will resolve as the same name anyway (e.g. "John XXIII" or "John XXV" or even "John MMXXIII" would still resolve as John XXIV). In the event that the Catholic Church starts allowing female priests, a female name with an obvious male equivalent in this list would resolve as it (e.g. "Paula" would resolve as Paul VII).