How many items are listed under viewers column when the bbd three weekly ranking was 1?
CREATE TABLE "series_2" (
  "episode_no" real,
  "airdate" text,
  "viewers" real,
  "bbc_three_weekly_ranking" text,
  "cable_rank" text
);
SELECT COUNT("viewers") FROM "series_2" WHERE "bbc_three_weekly_ranking"='1';