There are 10 total stars available in the game, once you have installed the Halloween update and have unlocked Cheats and Challenges by clearing 20/20/20/20 Mode.

Star 1 - Clear Night 5
Star 2 - Clear Night 6
Star 3 - Clear Night 7 (Nightmare Mode)
Star 4 - Clear Night 8 (20/20/20/20 Mode)

Once you have the above stars, then you can add specific challenges while clearing stages to obtain the remainder of the stars, as outlined below.

Complete Night 7 with any challenge active
Complete Night 7 with Blind Mode, Mad Freddy, and Insta-Foxy active
Complete Night 7 with All Nightmare active.