How much fat do you need for a bbl?

Fat grafting for a BBL is highly variable depending on donor site availability and goals for the procedure.  In general the volume will range from 500cc each side to 1500cc if you were to look at 1 standard deviation.

    The amount of fat that is available is only one factor.   Your goals as well as your selection of your surgeon are important as well.

The question of whether a patient has "enough fat" is relative to their goals and expectations. Most patients have adequate fat for transfer, however, adequate for transfer does not necessarily guarantee a particular result. There must be some correlation between expectations and reality in order to ultimately obtain a satisfactory outcome. 

"BBL" or buttock augmentation is an excellent option for contouring the hip and buttocks area. However, there are limits to what can be accomplished. The primary limits are:
-donor sites available for fat transfer
-patient anatomy
Patients often forget that their shape is defined by more than mere fat content. Bone and muscle structure also plays a critical role in defining shape. While fat transfer can add to these contours, it cannot create them de novo. Patients with narrow hips will be hard pressed to achieve a dramatic contrast in waist to hip ratio.
It is also important to remember that not all of the grafted fat remains (up to 50% resorption rate). Thus more fat will need to be harvested and processed than can be reasonably expected to survive long term. I usually tell patients that for a moderate volume change they should expect at least a 500cc per cheek augmentation (assuming 50% resorption this leaves a 250 cc augmentation). In order to achieve this minimum, patients must have 1000 cc for harvest after processing (some is lost in the process). Less than this can potentially lead to a result so subtle it is difficult to appreciate.
As always, discuss your concerns with a board certified plastic surgeon.