If you look at the Skull Quests, the amount of skulls you can get is 933. The max amount you can get from missions is 810. Therefore, if we subtract 810 from 933, we get 123. Divide that by 3 and we get 41. There will be 41 missions total in Night City, assuming there isn't a new difficulty mode added after Nightmare for us to go through. I thought this would be interesting to share.