How many rankings (timeslot) were there for the episode that aired on May 2, 2010?
CREATE TABLE "us_ratings" (
  "num" real,
  "airdate" text,
  "episode" text,
  "rating" text,
  "share" real,
  "rating_share_18_49" text,
  "viewers_millions" text,
  "rank_timeslot" real,
  "rank_night" real
);
SELECT COUNT("rank_timeslot") FROM "us_ratings" WHERE "airdate"='May 2, 2010';