CREATE TABLE table_72317 ("No. in series" FLOAT, "No. in season" FLOAT, "Title" TEXT, "Directed by" TEXT, "Written by" TEXT, "Original air date" TEXT, "U.S. viewers  ( millions ) " TEXT)

How many episodes were watched by 12.72 million U.S. viewers?
SELECT COUNT("Written by") FROM table_72317 WHERE "U.S. viewers (millions)" = '12.72'
SELECT COUNT("Written by") FROM "table_72317" WHERE "U.S. viewers (millions)" = '12.72'