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?

The amount of volume that can be added to your buttock will be limited by the amount of excess fat you have in your donor sites. Please visit with a board certified plastic surgeon to learn more about your options.Dr. BasuHouston, TX

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.