Arngeir has, through more study, become an increasingly... unreliable source. Honestly, the Greybeards keep buggering up. Calling the Dragonborn Ysmir? Hah! Seriously, though... We simply don't know. Presumably, there HAS to be more, or will be more, because Alduin still needs to beat one in order to devour the world. As i understood it, They use Ysmir as an title. I think they give the title to the dragonborn that leads the fight. They gave Tiber Septim the title and he was a leader of men. So when the dragonborn stands up against the dragons (s)he practically is leading the fight against the Dragons, the grey beards give the title to the dragonborn Yep, the dragonborns decide who gets to be called Ysmir. I think it just means you are a (dragonborn) defender of the north against any threat, not just dragons. After all full title is 'Ysmir, Dragon of the North'