CREATE TABLE table_16336 ("Season" FLOAT, "Episodes" FLOAT, "Time slot  ( EST ) " TEXT, "Season premiere" TEXT, "Season finale" TEXT, "TV season" TEXT, "Rank" TEXT, "Viewers  ( in millions ) " TEXT)

How many season premiers have a rank of #21?
SELECT "Season premiere" FROM table_16336 WHERE "Rank" = '#21'
SELECT "Season premiere" FROM "table_16336" WHERE "Rank" = '#21'