How many years did BBC One rank 20th?
CREATE TABLE "tv_ratings" (
  "year" real,
  "broadcast_date" text,
  "bbc_one_total_viewing" real,
  "bbc_one_rank" text,
  "bbc_two_total_viewing" text,
  "bbc_two_rank" text
);
SELECT COUNT("year") FROM "tv_ratings" WHERE "bbc_one_rank"='20th';