It is impossible to predict cup size becasue breast cup sizes are not standardized. Cup sizes vary by designer. So a C cup in one designer could be a B or even a D cup in another designer.
Since cup sizing varies so much from one manufacturer to another, it is near impossible to predict what cup size you'll have after surgery. Try to focus on the "look" instead of a cup size, meaning when you look in a mirror without a top on do you like the size that you are? Does it fit your frame? If the answers to those questions is yes, then whatever cup size that produces is the right one for you. Good luck.