CREATE TABLE table_21781578_2 (season_no VARCHAR2, us_viewers__millions_ VARCHAR2)

What was the number of season number entries that had US Viewers of 12.30 million?
SELECT COUNT(season_no) FROM table_21781578_2 WHERE us_viewers__millions_ = "12.30"
SELECT COUNT("season_no") FROM "table_21781578_2" WHERE "12.30" = "us_viewers__millions_"