What is the average amount of fat transferred to each buttock? And how successful is a BBL?

If you are at a stable weight with a BMI less than 35 (maximum) with no medical problems, you can likely have a substantial improvement with this procedure in which fat is removed from areas of excess and transferred to areas of deficit.  The results are almost totally surgeon dependent with regard to amount to be harvested and transferred.

Ideal candidates are at a stable weight,  in good health , and have excess fat in some parts of the body and too little in other regions.  Gaining weight before the procedure is to no advantage and when you lose weight again the fat cells will shrink just like the donor site fat cells.  Typically, the transferred fat results in a significant increase in volume of the body site being treated.

Brazilian Butt Lift transfer volumes depend on how much you can afford to give up from other areas. My average is 1000cc in each side. At first my fat retention (or take) rate was about fifty percent, and it has climbed higher as I got more and more experienced at it. See a board certified plastic surgeon, who operates at a certified facility and does a ton of BBLs and ask her, or him; "how many did you do last year and how much of the fat stayed six months later?" Don't settle for vague answers. Choose your surgeon carefully.

The amount of fat transferred will vary depending upon what is available to harvest, the goals of surgery, and what can be transferred. About 60-70% will usually "take".

The amount of fat transferred depends on how much fat is available and the skin laxity. About 30% of the transferred fat won't survive. I would recommend looking at before and after photos to assess the success of this procedure. Seek a plastic surgeon who performs many of these procedures, has great reviews and photos, and is an expert in this field.