How many were the show's average weekly ranking when it reached No. 2 in average nightly ranking?
CREATE TABLE "series_ratings" (
  "season" real,
  "num_of_episodes" real,
  "timeslot" text,
  "season_premiere" text,
  "season_final" text,
  "peak_audience" real,
  "average_audience" real,
  "average_nightly_rank" text,
  "average_weekly_rank" text,
  "average_yearly_ranking" text
);
SELECT COUNT("average_weekly_rank") FROM "series_ratings" WHERE "average_nightly_rank"='No. 2';