how many grafts would be necessary to achieve full coverage in this area?

When I examine and look carefully to your photos I have to say that you would have no benefit with HT for crown area, because there is not enough hair loss or sparse/bald area to restore. If you decide to have a hair restoration surgery, area will not be as dense as natural hair, also have a risk of permanent hair follicles damage. So, I do not advice you to have a HT op. I advice you to use hair long to cover the vertex or scalp micropigmentation ..