What is the number of series with production code 503?
CREATE TABLE "season_5_2008_09" (
  "series_num" real,
  "season_num" real,
  "title" text,
  "directed_by" text,
  "written_by" text,
  "original_air_date" text,
  "production_code" real
);
SELECT COUNT("series_num") FROM "season_5_2008_09" WHERE "production_code"=503;