How many original airdates did season 20 have?
CREATE TABLE "table1_22181917_2" (
  "series_num" real,
  "season_num" real,
  "title" text,
  "directed_by" text,
  "written_by" text,
  "original_air_date" text,
  "production_code" text,
  "u_s_viewers_millions" text
);
SELECT COUNT("original_air_date") FROM "table1_22181917_2" WHERE "season_num"=20;