How many audition city's are there with an episode air date of June 24, 2010?
CREATE TABLE "regional_auditions" (
  "episode_air_date" text,
  "audition_city" text,
  "audition_date" text,
  "audition_venue" text,
  "guest_fourth_judge" text
);
SELECT COUNT("audition_city") FROM "regional_auditions" WHERE "episode_air_date"='June 24, 2010';