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.