How many different original air dates did the episode number 6 have? 
CREATE TABLE "table1_13426649_1" (
  "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_13426649_1" WHERE "num"=6;