Get as much as you can get done at one time that will give you the density required but don't go overboard. Trust me you don't want to be going through the process multiple times, it is not pleasant for months afterwards. Go to Turkey or Belgium get a mega session and stay for a week if you need to or are worried about complications. That way your donor area will have healed by the time you leave and the grafts will have bedded in. A good surgeon will always leave enough donor for future use if the meds stop working but keep this in mind because as we age you will inevitable lose more hair.