How many writers are listed when the U.S viewers are 11.21 million?
CREATE TABLE "table1_24910733_1" (
  "no_in_series" real,
  "title" text,
  "directed_by" text,
  "written_by" text,
  "original_air_date" text,
  "u_s_viewers_millions" text
);
SELECT COUNT("written_by") FROM "table1_24910733_1" WHERE "u_s_viewers_millions"='11.21';