Hello and thank you for the question. This is of course, best answered in person, during a consultation with a board certified plastic surgeon. The amount is based on several factors: how much fat you have available to be removed safely, the amount of loose skin / soft tissue area of the butt, and your underlying health.
As far as combining this with a breast lift, that too is best answered in person. The safety factor becomes an issue with regards to your health and the planned duration of surgery. For many patients both can be done simultaneously. You should factor about 2 weeks off of work and about 1-2 months of no exercise during the healing process.
You should seek several consultations with board certified plastic surgeons before making any final decisions.
As always, best to be healthy, no smoking and make sure any health conditions you have are managed by your primary care md.
Best to you.