How many episodes had a production code of 5m13?
CREATE TABLE "table1_20967430_4" (
  "ep" real,
  "season" real,
  "title" text,
  "directed_by" text,
  "written_by" text,
  "original_air_date" text,
  "prod_code" text
);
SELECT COUNT("ep") FROM "table1_20967430_4" WHERE "prod_code"='5M13';