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 :)

Without seeing photos, it is hard to say if you are a good candidate at your current weight. I do not believe that people should gain weight
in preparation for a BBL. The fat that is gained often just goes away after the
surgery. This is why people complain that the fat didn't last. When choosing your doctor for this procedure, be sure to find
a Board Certified Plastic Surgeon who specializes in this procedure and
performs at least 6 to 8 per week.

Photos would really help in answering your question better. However, at your current weight for your height I do not think you need to gain any weight.  Consider reposting your question with photographs.