Is it safe to implant 6000 grafts in 2 days FUE technique?

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.All surgeries (no matter how minor) has risks. It is important to note that your donor area will always look "thinner" proportionate to the number of hairs taken out after a FUE harvesting surgery.  The hairs will NOT grow back from the donor area (where it was taken). It is for you and your doctor to discuss these risks versus the benefits.

A 6,000-graft procedure would be a very large hair transplant. Transplanting these many grafts at once would necessitate grafts being placed very close together and, in this situation, the blood supply may not be adequate to support growth significantly compromising graft survival.
Another reason for concern is that when harvesting, FUE yields about 20 grafts/cm2. A 6,000-graft procedure would require 300 cm2. Since the donor area is about 30 cm long, this would require a donor height of 10 cm, clearly extending beyond the permanent zone of the scalp of most patients.  

As my surgical mentor once said “when you start off your surgical career you make a large incision, as you become confident the incision becomes smaller and as you become more experienced (experience complications) your incision becomes larger again.’ The same applies for hair transplants. Initially, as a surgeon, one limits the number of grafts to 2000 grafts and then due to pressure of other ‘sham clinics’ offering more grafts at lower prices one may offer more grafts. But if one is honest implanting more than 2500 grafts in one FUE session increases complication rates, reduces graft survival and depletes the donor area. Nothing can replace experience and experience tells us that more than 2500 grafts in one sessions is a no no. 

While it is safe to harvest 6000 grafts in two consecutive days, there needs to be a careful examination of the donor area to see if it can support this amount of grafts. While we have extracted this amount of grafts in two days, it is the rare patient that has the donor supply to support this.

To be quite honest, those are ridiculous numbers of grafts-  unless he meant hairs, which would be around 2800 to 3000 grafts-  still a rather large number, but likely to be safely performed.Typically I limit my FUE procedures to 2200 to 2400 grafts (4500 or so hairs), with the ability to always return for a second procedure in the future.  These procedures this size will avoid the risk of damage to the donor area, and likely will improve the final results.  I will however harvest beard and or chest hairs when appropriate at the same time as these scalp hairs to provide further coverage.