FUE on crown. How many grafts would be required? (Photo)

Thank you for your question and for your photos. Based on your
photos provided, I would advise you to follow up with your surgeon to discuss further options. 

The goal of a hair transplant is to provide fuller coverage while preserving a natural appearance. An assessment takes into account such factors as the apparent thickness and curl of your donor hairs, the color contrast between your scalp and hair, your hair loss pattern, your age, and many other factors. The state of the art technique is FUE (follicular unit extraction), performed with a hybrid blunt punch that minimizes graft transection to less than 4% while allowing for the use of the smallest possible punches to minimize donor site scarring while maximizing regrowth.
With FUE, the back and usually the sides of the head will be shaved down. Sometimes patients have a partial shave in which just the lower half of the back of the head is shaved, which either can be covered up by somewhat longer hair above it or if you have shorter hair it looks just like you had a “fade” hairstyle.  A no-shave FUE technique is also possible.
In your situation, 2500 grafts would be recommended for transplant, given the donor area is viable, which you would have to consult a specialist to determine. 
However, the crown can often be a difficult area to achieve adequate density.
Gary Linkov, MD
Hair Specialist
Manhattan

Photos cannot determine if you would be a good candidate for surgery.  You already had surgery to understand its limiations.You need to have good communication with your doctor to understand what your end result will look like. 
Someone with your hair loss pattern may be perfectly happy with 1000 grafts, while others may not be happy even with over 3000 grafts.  You also have to realize one surgery may not be enough (as you lose more hair).  In addition, straight black hair on fair skin results in a thinner appearing outcome then someone with curly blond hair on fair skin.   I hope you can appreciate there are many factors to your end result than just a "number" of grafts.