what's the overall rank with rating/share 18–49 of 2.1/5
CREATE TABLE "weekly_ratings" (
  "num" real,
  "episode" text,
  "air_date" text,
  "rating" text,
  "share" real,
  "rating_share_18_49" text,
  "viewers_m" text,
  "timeslot_rank" real,
  "night_rank" text,
  "overall_rank" real
);
SELECT COUNT("overall_rank") FROM "weekly_ratings" WHERE "rating_share_18_49"='2.1/5';