How many different production codes does the episode originally aired on October 18, 1984 have?
CREATE TABLE "table1_2818164_2" (
  "no_in_series" real,
  "title" text,
  "directed_by" text,
  "written_by" text,
  "original_air_date" text,
  "production_code" real
);
SELECT COUNT("production_code") FROM "table1_2818164_2" WHERE "original_air_date"='October 18, 1984';