I can’t comment about bilateral
mastectomies, I only had one. But as @arpie says, there was remarkably little pain or discomfort. Arm mobility depends to a large extent on the removal of lymph nodes - digging about the axilla can upset nerves, cause discomfort and, often, temporary loss of sensation. But even then, it varies. I had 17 lymph nodes removed and had normal arm movement within 48 hours. Couldn’t tell cold from hot in my armpit for months though! Exercises will be recommended and are really worth doing. However the issues are different if you are having immediate reconstruction. I had none so others are better placed to advise in that case. Very best wishes for your surgery.