Dragons are very territorial apex predators. They build lairs on mountaintops, roost whenever they aren't hunting, and they hunt from the sky. With their size, wings and firebreath, there is no virtually no prey that they can't take down.
They hunt over grasslands, coasts, and mountain ranges - typically targeting large animals like bison, elk, deer, bears, walruses etc. For their prey, the only way to avoid a dragon is to run for cover and hope it doesn't spot you. For that reason, heavily forested areas make for poor hunting ground.
A dragons hunting territory is the minimum ecological area that contains enough prey to safely sustain the dragons bulk. A dragon cannot leave it's own territory without infringing on that of another dragons. Hunting territories are well defined, and fiercely protected.
I'm thinking that dragons would have to be sparsely concentrated over a continent, or else they'd destroy the ecosystem by overhunting. With their wings, their territory can be as far as they can fly, and can include land and sea. I'm wondering what sort of size in square miles their territory would actually be, and how many dragons could fit on a map.
Obviously it would vary depending on the terrain and abundancy of local prey, but I'm just after a standard ballpark figure.
Assume a fully-grown dragon has a 45 foot wingspan, and weighs around 2500lb.
Edit: I don't have any information on a dragon's food consumption or energy requirements, but I was hoping for an answer that makes reasonable estimations / best guesses. Assume the dragon is like a dinosaur, perhaps; it's reptilian, but warm-blooded and sustains a large mass.
Like any animal, it tries to conserve energy wherever possible and it tries to hunt using the minimum amount of effort. There is some magic involved for it's fire breath / flight, yet assume any magic is secondary and that it still needs to eat like any other animal.