vary depending on what you start with and whether you are having reconstruction at the same time. Your surgeon whom you choose to do this for you should be able to clearly show you what scars you have to accept. And then you have to discuss whether your nipple should be 'saved' and I would suggest you at least consider keeping it as nipples are very hard to reconstruct with results that hold up long-term.
This question should be directed to your breast surgeon. I am assuming that your mastectomy will be skin-sparing mastectomy.. Most times, the scar is in the middle of the breast, where the nipple/areola used to be. Some surgeons may place scar on the bra line. You should talk with your breast surgeon prior to your surgery to have all your questions/concerns addressed. In surgery, there are several different ways to do the same type of surgery. Best wishes with your surgery.