I dont think it is difficult to add more density with another op. People do it all the time.
I think an average density of 40 grafts per sq cm should be sufficient to provide a good illusion of density. These grafts would be concentrated more near the hairline, say 50-55 grafts per cm, that the crown, say 25-30 grafts per cm.
I am sure the doctor would take care of this aesthetic design so dont worry.
My guess is that a typical norwood 5/6 would have 175-200 cm2 reciepient area. For average density of 40 grafts per cm, one would need 7000-8000 grafts.
These are just my thoughts. Consult with Dr.Bhatti as he would be able to give you a clear personalized plan to achieve what you want.
Please share your pre-op pics too...