Can you tell me how many x rays they take for root canal treatment of a lower molar?

Depends: Generally a preoperative x-ray (one straight on and one at an angle), if the dentist is using a root zx apical locator then a cone picture, if not then a file film and a cone picture with a final picture showing the canals filled and one of the build up. Even with all the x-rays these days the dose is so low standing outside or going to the airport security doses you more than the rct pictures.