CREATE TABLE table_72994 ("Series #" FLOAT, "Season #" FLOAT, "Title" TEXT, "Directed by" TEXT, "Written by" TEXT, "Original air date" TEXT, "U.S. viewers  ( millions ) " TEXT)

What series number garnered 20.64 million viewers?
SELECT "Series #" FROM table_72994 WHERE "U.S. viewers (millions)" = '20.64'
SELECT "Series #" FROM "table_72994" WHERE "U.S. viewers (millions)" = '20.64'