Is it better to wait and have one large FUE surgery (6000 grafts) as opposed to multiple 2000 sessions?

Planning your surgery is between you and your doctor after an examination and understanding your goals.  There is no right or wrong / better or worse.  6000 grafts seem like a very ambitious surgery,

It would be better to have 2 procedures of 3,000 grafts FUE for the best yield. Be sure to choose an FUE surgeon that has a lot of results of 3,000 graft plus results. There are not a lot of great fue surgeons that can do that many.A great first step would be an online consultation. Often you can submit photos to top hair restoration surgeons for a complimentary evaluation. This should help lead you in the right direction. Having multiple consultations with different doctors cannot hurt. 
Lastly, Do not let location choose which doctor you go with. Rather then choosing your hair restoration doctor based on who is close to you, you should research the top clinics and go with who's results you like the best. Many of today's top clinics will reimburse you for travel cost, and you can leave for home the very next day after the procedure, so travel and location should not be a top consideration when choosing your doctor. The result you receive will be permanent and there for life, so don't be afraid to take a short trip to go with your favorite clinic.

Large FUE sessions are btwn 2000 to 2500 grafts.  This is due to several reasons. First, a session greater than 2500 grafts makes for a very long day.  Pt become understandably restless and tired despite our best efforts to make them comfortable in the chair.  Grafts from the permanent zone are carefully selected and that takes time.  Going for 6000 at any one time could give the appearance of over harvesting. Lastly, procedures performed in several smaller sessions over time allows for fine turning in the recipient area (if need be). 

In most of the cases,Fue hair transplant with more than 3000 grafts end up in severwly depleted donor area....multiple sessions are  better than one go..

donor depletion is one of the draw backs for larger fue session over harvesting with out proper calculation of donor availability. Your doctor needs to be experienced and careful how mauch hair is harvested without making the donor look too thin. 

By limiting your FUE hair transplants to roughly 2000 grafts, you are ensuring a higher take rate for the transplanted grafts as well as not running such a high risk of completely over-harvesting the donor area. I always recommend keeping each FUE session to around 2000 grafts instead of the higher megasessions. 

A 6000 graft FUE session will most likely cause donor site depletion and a see-through donor area unless you are the 1% of the Caucasian population who have unusually high donor densities (see link)