given their size, dragons would need an abundance of food, if they are active. This would mean they would require territories as G-Flex says, in a similar manner to other large predators. One would expect dragons to defend their hunting territories against other dragons and competeing predators. You would expect the population of dragons to be capped at the number of available hunting territories, + any in hibernation. Hunting territories would be quite large, and even larger in poor biomes, such as deserts etc. In game terms this could mean you get dragon vs dragon combat if the population is too high, or dragon verses city combat if they become desparate for resources. Over population should increase dragon against sentient attacks (greater fun).
From fiction (may not be true of DF universe) we also know that dragons hibernate, and like caves. If they like caves they cannot be cold blooded (or at least it seems less likley). hibernation would allow them to survive longer if they had no territory but they would need somewhere secure against pesky humans, and other predators.
We also know they have a similar lust of shiny objects like the common magpie, and in some fiction, and i think DF, dragons are sentient. Even at lower populations dragons would have to attack sentients to claim shinies. they already appear to do this.
As for hatching eggs, they would need somewhere easily defendable, so a cave, empty fortress, or mountain top. I dont know what temperatures eggs need to survive, and whether a mother defending her egg(s) is in semi hibernation, or needs to hunt. Theres also the question of the males role, does the male just mate then leave, or is he an active parent? Also which is bigger, male or female dragons?
Luckily in df there will always be one population control for dragons.. Adventurers, the most mindless and ilogical predators of them all.