What was the number of season number entries that had US Viewers of 12.30 million?
CREATE TABLE "table1_21781578_2" (
  "season_no" real,
  "title" text,
  "directed_by" text,
  "written_by" text,
  "original_air_date" text,
  "production_code" text,
  "u_s_viewers_millions" text
);
SELECT COUNT("season_no") FROM "table1_21781578_2" WHERE "u_s_viewers_millions"='12.30';