Hi there-
Without examining you myself, it is very difficult to answer this question for you definitively, but I do have a few thoughts (having successfully performed a great many of these procedures myself) that hopefully will help you.
Most importantly, you should realize that the goal of a BBL is to establish new proportions between your thighs, back, abdomen, waist, and buttocks, NOT to simply make the buttocks larger. It is the combination of a reduction on the prominence of these other areas, in combination with the increase in volume of the buttocks, that produces the pleasing change in appearance we associate with a good outcome from this operation.
This volume required will of course also depend on your body type and proportions now- so it's not possible to tell you that any particular volume would achieve your goal. Let's use breast implants as an analogy... obviously a 300 cc implant would look different in a patient who is very thin, has little of her own breast tissue, and has never had children than it would in a patient who is a bit heavier and has had a couple of kids, right? This is not that different- what matters is not the absolute volume transferred, but how effectively it is REDISTRIBUTED. This, in turn is all about experience with this procedure.
So the question is not how much fat would need to be placed to achieve your goal, but really, the question should be, "Will the combination of the fat removal from the areas planned and grafting to the buttocks achieve the shape I am looking for?". The second most important question should be, "How many of these particular operations has my surgeon performed?".... They should be able to show you photos of patients they have personally performed this procedure on, which show results you would be pleased with.
