The number of sessions depends on:
1. What is the primary cancer such as breast, lung, colon...
2. Whether you have been treated before in the area of metastasis and it is coming back (every tissue has a limit as far as how much radiation it can tolerate without creating problems for you; you can only get a smaller amount of radiation if you were previously radiated in the same area.
3. How big the metastasis is and what the radiation oncologist has to do (be creative about irradiating a large amount of tissue without damaging surrounding tissue too much).
4. What method is used to treat the metastatic cancer (small doses given 3-5 days a week over 3-6 weeks vs large dose to a small amount of brain tissue in 1-2 doses).
You can ask all those questions to the Radiation Oncologist (doctor who treats your metastatic cancer with radiation). I am a diagnostic Neuroradiologist who reads your MRIs, CTs of the brain, spine and neck, of cancer patients, but does not treat the metastatic disease that I diagnose.