As long as you've got the battlemap space and the environs are big enough, 3e handles 12+ opponents just fine. What can't handle it is the encounter level estimation and judging, algorithmically, whether or not it's a good encounter.
I like (and have used) the swarm/mob approach to this. The "fireball survival" thing doesn't bother me - I just assume that the tightly packed mob is provided some shielding from explosions by it's density.
