What's the maximum amount of fat that can be put back in during a bbl? Or is there not a guideline in the US?

There are several factors determining how much fat can be transferred to the buttocks during a BBL, such as the amount of donor fat available, the desired size of the buttocks, and body type. Please consult with a board-certified plastic surgeon to discuss your options.

I think that most surgeons in the states would not take out more than 5 liters of fat in an outpatient facility and in some states its even less.   As for putting into the buttock, there is a delicate balance between what can be injected and what will 'take."  I don't think that there are any scientific studies regarding this. I think for most surgeons it's a "feel" of what  is felt to be not too tense. I have put 1000/ side and sometimes more depending upon where they need it.