What StevenE said.
From the bottom of page 16 of the rule book:
A player may only play a maximum of one Dragon card during your turn. A player may only play a maximum of one Dragon card during your opponent’s game turn. As a result, during the course of any single game turn, there can only be a maximum of two Dragon cards coming into play, one from each player. In case of contradiction between the effects of two Dragon cards played in succession, the second card trumps the effect of the first one.
In case of conflict between the basic rules in this book and a Dragon card, the rules of the card take precedence, however, terrain movement and battle restrictions will still apply.