The number of dragons depends on the difficulty level. Also, their numbers increase in the same way as those of regular neutral creatures. Therefore, it is easy to gather a hundred of them for your hero. I had to stay longer in this mission because not all dark magic and summoning spells were available in the castles. I had to obtain them in the treasure chamber and the pyramid. Otherwise, it would be difficult for the hero to defeat a very well-leveled Godric.