After 14 months of hair transplant, should I stop the Minoxidil or not?

You need to ask your doctor.  Some doctors advocate using minoxidil after surgery.  Some doctors do not.  There is no absolute rule.

It will be advantageous for you to continue with the medication in order to maintain your existing hair. There have been numerous reports of hair loss resuming when patients stop using minoxidil. 

We advise our patients to use Minoxidil even 14 months after the hair transplant as Male Pattern Baldness is a progressive condition. So, in order to delay the further hair loss, you should continue. However, if you are of a higher grade of baldness, there are no more hair to be lost and you have got only transplanted hair on your scalp, then you can even stop Minoxidil as the transplanted hair will not fall after stopping medicine..

Very impressive that your doctor managed to give you 6500 grafts. Having committed to a hair transplant, I would suggest you keep on the minoxidil to try and hold on to any miniaturising hairs within and around the transplant zone.

In theory this does not have an effect on the new transplanted hairs just on the hairs you had before the transplant.  So really, it affects the final overall look but not the actual transplant