On this guide page, you will learn how to get the ending with the number 8 in Stanley Parable Ultra Deluxe.
To unlock this ending, you have to complete the Prologue first (see the New content - how to unlock? page to learn how to do this). There, you have to agree to the narrator's proposal to create new parts of the game by adding a subtitle and another number.
You should reset the game several times until the number will be 8. Start a new game, and you will see a room with only a button with the number 8. Keep pressing it until you exhaust the narrator's dialog lines.
To play Stanley Parable again and continue to explore the offices, you have to turn the game off and restart it to change the "sequel" number. Otherwise, the room with the button will appear every time.