If was evaluated at ~9000 FUT grafts, what's the best estimation for total number of FUE grafts?

The success or results of a hair transplant surgery is not about "how many" or "the number of grafts".
The results depend on a variety of other factors such as how long you keep your hair, how you style your hair, the hair color to skin color contrast, curl of your hair, the thickness of your hair.  If you have very fine straight hair that you keep very short, a hair transplant surgery will NOT look good.
The overall success of a hair transplant surgery depends on: (1) artistic and technical skill of your doctor and team (2) communication between you and your doctor to establish a realistic goal.

The average male has approximately 11,000 follicular units. Of these, the most that can be harvested is 6000.The average FUE case will involve approximately 2000-2500 grafts. If more are needed, these procedures should be staged/staggered.As always, discuss your concerns with your board certified plastic surgeon (ABPS) with experience in hair restoration.

Is your Hair restoration surgeon talking about available graft, hairs, or the total amount available for grafting over several sessions.  Doing 9000 grafts at one sitting in my opinion as a board certified plastic surgeon is dangerous to the existing scalp and increases the risk of post op complications due to swelling popping and blood supply.  If you are talking a 3 sessions then I would be fine.  Just curious if you have tried laser light therapy and PRP as a means to maintain as opposed to grow hair.  You might want to speak to several doctors before making a decision as the most grafts I have done in one sitting of FUE using the SmartGraft is 3500 and that was a WHOLE day.

Graft numbers are very deceiving. A graft is a piece of skin. We could theoretically take an unlimited number of pieces skin from the scalp. The smaller pieces we take the more grafts there will be. However, the goal in hair restoration surgery isn't getting grafts (pieces of skin), it is to get hairs for coverage. Therefore, we should estimate total donor capacity in number of hairs, not grafts. This requires a live examination calculating donor density, skin laxity, surface area etc. Even then, we would not be able to give a conclusive number; this would be a very rough estimate, since there are many parameters that we can not fully control, like hair loss progression, transection rates during surgery, recovery of skin laxity after the first procedure, the appearance after a certain amount of hairs are removed from the donor area. This is a very complex task and giving quick answers and exact numbers should be seen as a sign of unprofessionalism.

The highest amount for anyone is about 6000 grafts that can safely be harvested and used in a hair transplant. I would say that doing 9000 grafts at any time with any procedure is a bad choice and impossible. But to make a long story short, you most likely have roughly 6000 grafts and I always suggest doing 2000-2500 grafts at each session to maximize results. 

The average Caucasian has about 11,000 Follicular units of which at least 6000 can be harvested for a hair transplant.  The limit on FUE itself, is around 3600 in this typical Caucasian male because more could produce donor site depletion (see web reference below)You must have misunderstood the reference to 9000 FUE as he probably measured your donor density at less than average and told you that your total donor supply's follicular unit count was 9000. I can't believe that any competent doctor would consider 9000 FUE as it would be a deforming surgery.