Breast Reduction 34N cup how many grams would I need to remove to be a C-D cup? 180lbs 5'5.

The volume removed from surgery does not always correspond exactly to cup size because bras are not all made the same.  I will say if you are quite large, then getting down to a "C"ish may be difficult without  potentially compromising shape and viability.

This something you will discuss with your surgeon at your pre-op visit. This surgery is an indivualized procedure.   The amount of weight removed does not corelate with a specific cup size.   

Thank you for the question and congratulations on your decision to proceed with breast reduction surgery; this operation tends to be one of the most patient pleasing operations we perform..
There is no direct correlation between the amount of tissue removed and the ultimate cup size that a patient will wear after breast reduction surgery.  There is simply too much variability among bra cup manufactures to make it a useful metric when it comes to preoperative communication and/or post operative predictions.
Before undergoing the breast reduction procedure it will be very important to communicate your size goals with your surgeon (who you should choose very carefully). Most patients wish to achieve enough of a reduction to help with their symptoms while remaining proportionate with the remainder of their torso. 
With the goal of improving communication with my patients I find the use of photographs of “goal” pictures (and breasts that are too big or too small) very helpful. For example, I have found that the use of words such as “natural or proportionate” or “C or D cup” means different things to different people and therefore prove unhelpful. Also, as you know, cup size varies depending on who makes the bra; therefore, discussing desired cup size may also be inaccurate. 
Once you have communicated your goals carefully, your plastic surgeon will be able to provide you with an estimation of how much tissue will need to be removed, in order to achieve your desired outcome.
Best wishes with your breast reduction procedure; hopefully you will be very pleased with the outcome of the procedure performed.