Name the number of series for 19
CREATE TABLE "table1_23958944_6" (
  "no_by_series" real,
  "no_by_season" real,
  "title" text,
  "directed_by" text,
  "written_by" text,
  "original_air_date" text,
  "production_number" real,
  "u_s_viewers_in_millions" text
);
SELECT "no_by_series" FROM "table1_23958944_6" WHERE "no_by_season"=19;