This is a field that has changed a lot in the last decade. The amount of radiation needed to take standard dental x-rays has gone way down, the film technology is continuing to change allowing us to significantly reduce exposure time. During our lives we are continually picking up background radiation from the earth and the atmosphere around us. We calculate the amount of radiation that an x-ray uses in BERTs (Background Equivalent Radiation Time) in units of days adn hours. The standard 2 bitewing xrays that you have at your yearly check-up, are the equivalent to the amount of radiation you pick up in 14 hours. If your dentist is using digital x-rays your exposure is even less.
Another way of calculating radiation is in BED units (Banana Equivalent Dose) Bananas contain natural radiation because of the isotopes that they contain. Dental X-rays are somewhere around 50 BED. Close to eating about 50 bananas. Contrast that with the natural radiation from potassium isotopes in your body over a year which is about 3900 BED, or a flight from New York to LA which is around 400 BED. Basically you could have 8 sets of x-rays.