Is getting a lot of x-rays a risk factor for getting cancer?

YES. There is certainly a risk of getting cancer from low-dose x-rays, and it increases with more x-rays you receive. The risk of radiation therapy is slightly higher, as the dosage can be multiple folds higher than the low dose x-rays. However, the risk of cancer is extremely small. Overall, the benefits almost always outweigh the small risks of both low-dose x-rays and ionizing radiation therapy.
Yes. Radiation is a known carcinogen. Risk assessment came from situations like the japan nuclear bombings. Medical radiation is designed to minimize the level based on the type (mammograms are very low and bone scans are high). The frequency in which we do them are meant to give the benefit we want and not do a lot of x-rays on people. The benefit can outweigh the risks. But it can be overdone.