How many rankings were there for the season with a finale on August 20, 2011?
CREATE TABLE "seasonal_ratings" (
  "season" real,
  "episodes" real,
  "timeslot_et" text,
  "season_premiere" text,
  "season_finale" text,
  "tv_season" text,
  "rank" text,
  "viewers_in_millions" text
);
SELECT COUNT("rank") FROM "seasonal_ratings" WHERE "season_finale"='August 20, 2011';