The first 2 scenarios can be played fine with 2 players but the last scenario requires breaking up into teams. So for that one I would suggest 4 players minimum.
I'm not sure if Unlock is at best with 2 players though. Your apt to run into more roadblocks (and thus ask the app for hints). With more players, you can solve puzzles more quickly because there's more chance for someone else to brainstorm a solution. However, at the higher player counts, players might not get to see everything in the rush to solve the escape room in under an hour. The puzzle might already be solved so your input wouldn't be needed.
When you say last scenario do you mean a specific one of the 3 retail releases or does each release have multiple scenarios?