Multiple voices

Do someone know how to make the effect when from one vocal it makes like multiple voices? Examlpe here https://www.youtube.com/watch?v=TciYA70POj0 from 2:25

Well a quick and easy way to simulate that would be to copy the vocal track multiple times and place them each at very close offset spaces.
Or if you have the money, some FX have the ability to do that for you, even changing the pitches for you, to give you variety.
Izotope's Nectar does that well for me.
Also, judging from the way the track you uploaded uses it.
You might want to use a small amount of reverb, and if you're the one recording the tracks be sure to make different takes on it. You don't want it to sound entirely the same.
Good luck!

There's a lot of vocal stacking used at 2:25. Many vocal harmonies are layered below the Lead vocal. To create these kind of harmonies you can use "Melodyne" for example and change the melody of the vocal (you need to know about chords and harmonies though). Another way is to use a harmonizer VST (e.g. Eventide Harmonizer). If you just wanna have the same vocal "duplicated" you can use Chorus or Doubler Effects (check out Waves Doubler). But you won't get the same results as if you had recorded several original Dubs for the Lead Vocal.
Moreover it seems to me that some of the backing vocals in the track were recorded in a live room with a mic in the mid (atleast it sounds really authentic).