Can I get my singular veneer to match the rest of my teeth? Do I need more veneers or change the color of this veneer? (Photo)

The most difficult thing to do in dentistry is match one front tooth with another. 
It is much easier to do 2-4 veneers as stated below but you want to make sure that the lab that the dentist is using is good and that theyll make them look natural where we see veneers that are bland and have no life like translucency. 

Hi:For best result due to high translucency of your teeth, its best to do 2-4 upper porcelain veneers.Best,Dr. Maddahi

The veneer can be redone to match the other natural teeth much better. It will never match 100% unless you have more veneers done, but it can be redone to be much more esthetic. 

Matching one single veneer to the remaining natural teeth is one of the hardest esthetic problems in dentistry. I believe this single veneer could be done to match a bit better. More whitening may help but still unlikely to satisfy your desires. If you love the looks of the one single veneer then having the natural teeth veneered to match, could be an option. That means 7 or so more veneers. Ideally you would align your teeth (braces/ Invisilign/ other) a bit and have a well trained esthetic dentist match the single veneer. Keep smiling.