I hope I'm not over-anolyzing, but the narrator in the teaser said: "...we never knew THEY existed..". So at least 2. ^^
Don't think you'll be able to ride them, but not ruling that out considering that we don't know anything about the game or the 'destiny' of the playing character at this point.
Since I have the impression that they are very large, I suspect that they're not too common. Perhaps there will be random encounters with Dragonlings.
On a side note, I'm actually not a big fan of Dragons anymore. They're so over-used in fantasy that I grew tired of them.
I hope at least that you can communicate with them and that they're not generic mindless beasts.
Having a story like: *Oh, no a dragon burned down Rifton!* ...and then you have to go slaying one dragon after the other would svck beyond reason.