On average, annual screening of 100,000 women aged 40 to 74 years was projected to induce 125
breast cancers... leading to 16
deaths... relative to 968 breast cancer deaths averted by early detection from screening."
However, that's an overall guess based on averages. To zero in more:
"Women exposed at the 95th percentile were projected to develop 246 radiation-induced breast cancers leading to 32 deaths per 100,000 women.