I quit smoking but sometimes dying to smoke...If I smoke. Once or twice a month would it be harmful?

Reduced smoking. Not smoking is the best. Most consider above 5 cigarettes a day to increase the damage to the lungs, and not let the lungs heal. Less than 5 per day does not cause damage, and may allow some healing. Smoking a few per day, week or month, continues the craving for nicotine, and thus the addiction. The less you smoke the better.
Yes. There is no level of smoking that is safe. I also expect you would resume your normal smoking habits within a short time.