I think there should only be very few dragons running around and what I'm thinking of doing with them is that there are like the ancient protectors for the different races and what not. As in like Gold Dragons were the protectors of the elves. But over the years the Ogre Lich has hunted down and killed most of the good ones and he uses the evil ones that are the protectors of the ogres as generals in his armies. Since he has done that the other dragons have come down to help out the other races in there war against the ogres but an old truce of the dragons prevents them from using their full power in front of the lesser races.
On the number of dragons I think there should only be one left for each race and the ogre lich still has one to fight each of the other dragons on a one to one bases