How many cigaretts a day are safe to smoke?

None: Smoking is bad habit and major risk for cancer heart disease and poor dental hygiene

For safety, none: For less risk, less cigarettes. It is not likely to be a one-to-one relationship since each smoker is different in many ways. Genetics, other environmental factors, depth of inhale, other illnesses, time of day, quality of the tobacco and filter of the cigarette etc., etc. All matter. The fewer the safer. Some do say that under 5 per day is a minimal risk. You have to decide how to live.