What is the series number that has a production code of 50021–50025?
CREATE TABLE "season_5_1992" (
  "number_in_series" real,
  "number_in_season" real,
  "title" text,
  "directed_by" text,
  "original_air_date" text,
  "production_code" text
);
SELECT MAX("number_in_series") FROM "season_5_1992" WHERE "production_code"='50021–50025';