Deep Dreams has a total of 3 different endings, all of them with their respective requeriments to trigger them. All the game's endings are listed below.
Unlike most YNFGs, there's no need of dropping your alters in any specific place since there is no way of getting rid of them, but getting all of them is still mandatory.
Ending 1
Requirements: Get all 12 Alters.
Description: Once you get every Alter, wake up and open the door Mariana refused to go through, now you'll be able to leave. Once outside the room, go to the rightmost door (The other ones won't open) and open it. Mariana will finally be outside of her house, leave through the only way.
Ending 2
Requirements: Get all 12 Alters and see the Tetsuko and Tetsuko's Past events.
Description: Once you get every Alter, wake up and open the door Mariana refused to go through, now you'll be able to leave. Once outside the room, and open the door with the pink cushion on the ground. Inside there will be the exact same room where you encountered Tetsuko in the dream world, Interacting with her will play a small cutscene where the two girls look at a door, open it and leave together to an unknown place.
Ending 3
Requirements: Get all 12 Alters and see the Reactivation event.
Description: Follow the same steps as before, but now go through the third door instead. After going up the stairs, Mariana will end up in the rooftop of her house. Despite being awake at this point, Julian will be there somehow, sitting on a bench, interacting with Julian will show a small cutscene where Mariana sits next to Julian, and fly away together after a few seconds.