You also have to consider what this game was made for - namely that you play it with a friend or two. As in, if you're on Discord or Skype or whatever, and you're talking, and thinking like "what could we do" - here's a game you can play.
Because of that, the earlier dungeons can all be done with two, the later with three players. You can go into the highest level dungeon, set it to hard mode, and you'll (number-wise) be fine with two friends.
I actually encourage to play that way - that way you figure things out yourself (which is very possible - all boss mechanics are explained through in-game messages, (de)buffs and combat log entries, and there's people who have done that before), and don't just get dragged through dungeons in an expendable low-risk role by vets. ;)