Bra Size from Additional 120 Cc After Breast Implants?

Hello! Thank you for your question. In general, implant size does not
correlate with bra cup size. The cup size itself will vary from
manufacturer to manufacturer as well as who is doing the actual
measurements. Thus, cup size is never a reliable indicator for your
breast size. I typically encourage my patients not to communicate her
desires in cup size but more on the actual look and appearance. Good
communication between you and your surgeon of your expectations is
warranted - choosing your surgeon wisely is the first step. Discussion
of your wishes and having an honest and open dialog of your procedure is
mandatory (e.g, implant type, incision, placement, lift, etc). I have
found that photographs brought by the patient is helpful to get a
visualization of the appearance you wish for in terms of size, shape,
fullness, etc.  Also, preoperative sizing may help both you and your surgeon.   In addition, your surgeon's pre and postoperative
photographs should demonstrate a realistic goal for you. Once this has
been accomplished, allow your surgeon to utilize his/her best medical
judgment during the procedure to finesse the best possible result for
you after preoperative biodimensional planning and fitting the right
implant for your breast width. Hope me that this helps! Best wishes for a wonderful result!

Do not get fixated on a cup size because all bras are mad differently.  See how the volume looks in a loose bra and the clothing style you would like to wear. 

The problem with trying to explain breast size as it relates to bra size is that no two bra manufacturers use the same sizing. There isn't really any standard in women's clothing. For example the more money you pay for a dress the smaller the size will be. Strange but true. Unlike men's clothing which is based on actually measurements.
So it's not the bra size that matters but how the breasts look compared to the rest of the body. I look for proportion of the breasts in relation to the shoulders, hips and buttock. Go to your surgeon with a sports bra and add 120cc sizers and see how you look. Forget the bra size. You can always find one with the number you're looking for.