CREATE TABLE table_12570759_2 (written_by VARCHAR2, us_viewers__millions_ VARCHAR2)

How many episodes were watched by 12.72 million U.S. viewers?
SELECT COUNT(written_by) FROM table_12570759_2 WHERE us_viewers__millions_ = "12.72"
SELECT COUNT("written_by") FROM "table_12570759_2" WHERE "12.72" = "us_viewers__millions_"