I've reviewed the French localization as well, which matches the Japanese definitions.

The same three strings with the French localization and English translation are:
  • Bonus de participation / Participation bonus
  • Participants actuels / Current participants
  • Requis / Required [number of participants]

This, in addition to observed behavior with how this number increments based on number of characters who have bought tickets (and not the number of tickets themselves) leads me to believe that the localization referring to the "tickets" is inaccurate with how the system works.