How long after giving birth can you get a BBL done. Why do some surgeons say 3 months after brestfeeding and some say 8 months?

I would recommend you wait at least 6-8 months before having surgery, every surgeon has a different protocol go with the one who makes you feel comfortable just make sure he or she is a board certified PS.

It is best to wait at least six months so that your body can recover from the changes caused by the pregnancy to avoid complications and improve your recovery time.

Stability is the goal. It is always best to exhibit restraint in the post-partum period. This is due to the fact that your body is changing dramatically during this period. The hormonal fluctuation can alter size and shape widely during the immediate post-partum period and can continue through the period of nursing. Body contouring during this period is the equivalent of hitting a moving target.
The optimal candidate for surgery has reached a healthy stable weight. In the setting of a recent pregnancy and delivery this stable weight can take some time to reach. A general rule of thumb is at least 6-9 months post-partum. If you choose to breast-feed through the first year of life you should allow at least three months following the discontinuation of breast-feeding before considering any procedures.
While there may be variations in the recommendations from one physician to another, the goal is the same...stability. Thus a delay of 9 months or 3 months post cessation of breast feeding (whichever is longer...will ensure that you have reached a plateau.
As always, discuss your concerns with a board certified plastic surgeon (ABPS).