Depends really
Solo is a one or two hero game for me. One if I want to push for a level 6 hero, say a DL for infernal. Two if I want to lower level, but 2 good heros for my army.
2v2 generaly 1 or 2 heros. 2 heros almost always however, cause solo heros arn't as useful in 2v2 games.
3v3, 4v4 is generaly a solo hero for me, simply cause like above I want to get my hero to elvel 6 fast cause in team games ultiamtes are really useful.
FFA I ALways get 3 heros, simply because you have 3 heros gaining insane amounts of xp. I got 3 level 10 heros in a game once.