How much weight would I need to put on for a bbl?

Hello dear!
Thanks for the question and provided information as well.
Your BMI is within the recommended range, but unfortunately, without pictures, it is very difficult to give you a proper answer. 
In general, you need to be physically healthy and at a stable weight, have realistic expectations, be a non-smoker and your BMI needs to be 30 or less.
l recommend you to make an appointment with a board certified plastic surgeon to talk about your goals and anything you need to know.
Good luck :)

Thank you for the question.To best help you, please send in some pictures in order to determine which procedures and techniques would be best for your case.Dr. Campos