How many acts are there in Diablo 3? There are total of 4 acts (act 1, act 2, act 3, act 4) in Diablo 3. Each act has a final boss and you'll have to defeat the boss in order to proceed to the next act.
The boss for act I is Butcher, act II is Belial, act III is Azmodan, act IV is Diablo. After you've completed all the four acts, you can proceed to the next level of difficulty, and there are total of 4 different difficulty levels in Diablo 3 as well (normal, nightmare, hell, inferno).