BioWare has revealed some stats about dragons in Dragon Age: Inquisition on the game's official Twitter account.
2,602,377 dragons have been killed to date, with the most-killed being the Fereldan Frostback, at a lowly level 12. The least killed is the Highland Ravager at level 23. It's not too surprising, a completionist playthrough will only get players to level 24 or so.
While the majority of the game's dragons are found in Orlais, only 58 per cent of dragons in the game have been killed there. In general, dragons are found in the more remote regions of each area, and some, like the Abyssal High Dragon, can only be found after completing quests.
You can find the strengths and weaknesses of each dragon here.