CREATE TABLE table_18728 ("#" FLOAT, "Title" TEXT, "Directed by" TEXT, "Written by" TEXT, "Original air date" TEXT, "Production code" TEXT, "U.S. viewers  ( millions ) " TEXT)

How many different original air dates did the episode number 6 have?
SELECT COUNT("Original air date") FROM table_18728 WHERE "#" = '6'
SELECT COUNT("Original air date") FROM "table_18728" WHERE "#" = '6'