The dragon only attacks once at the end of your turn when refilling the market. Even if two cards come out with dragon symbols.
However the person who left does their own dragon attack in place of their turn. Since they can no longer buy cards it should only attack once (with bonus cubes indicated by the step in the track they're on). So at most one attack per turn, you're just combining multiple turns....
If you're close to death, don't buy cards or you risk additional attacks.
This is excellent advice.
Also good life advice.