Guys, how many heroes do you usually take? I always take two... one as a tactician, the other as a mage...
P.S. And sometimes it's still interesting to play against the AI... when it gathers a large army and casts something interesting.

It all depends on who you're playing against and which faction you're using. The number of heroes also depends on these factors. For example, I often play with just one hero (if possible), as it's convenient. If I'm playing as Necropolis, I definitely won't go with just one; I'll take some vampires as well. If I'm playing as Haven, then I need an army (if the hero doesn't have good combat and leadership skills). If I'm playing as Inferno, then one hero is more than enough. The destructive power of this magic will leave no one on the battlefield.
At the beginning of the game, I always use just one hero, because if there were two, the experience would be divided between them. I prefer to have one good hero rather than two mediocre ones.