They said you're able to fight 2 dragons at once, and considering you can also use a shout to call a dragon to assist you I'm guessing they have atleast tested it with 3 dragons in 1 instance. (This is just an example of how much we can see on screen)
Todd also mentioned that at one point they thought of making it so you could recruit everyone as your companion.
Expect much larger scaled battles than what we've seen in the past.
Todd also said that recruiting everyone as your companion was scrapped after a week, haha. I can see why that would be a bad idea.. just like in Fable "Follow." Kinda loses its meaningfulness at that level.
He also said, same interview I believe, that when he's bored he'll just summon a bunch of dragons to ransack a town, So I assume the dragons will work well with many NPCs running around and quite a few fiery beasts on location...