Most women don't get breast cancer, the 1 case in 3000 (again I don't know the exact number, but this is reasonable for range) caused by xrays includes screenings of all women, including those who never get it.

However if you are a doctor trying to figure out how often to screen women, the danger if xrays is a strong reason to not do it too often. Daily screenings would catch breast cancer a lot sooner on average, but just isn't worth the risk even if the screening was otherwise free.