In how many different audition cities were the Bridgestone arena auditions held?
CREATE TABLE "regional_auditions" (
  "episode_air_date" text,
  "audition_city" text,
  "first_audition_date" text,
  "audition_venue" text,
  "callback_audition_date" text,
  "callback_venue" text,
  "golden_tickets" text
);
SELECT COUNT("audition_city") FROM "regional_auditions" WHERE "audition_venue"='Bridgestone Arena';