Really hard to guess but I picked 8. He has the skillset he just needs the mental focus and obviously to continue the progression of his game overall. Sometimes I feel he gets slightly vexed by the pressure. Not much but slightly.
Hoping for him to become a David Villa type striker. 20+ goals a year consistently for a decade. 300 career goals for club and country is his cieling I think.
7 or 8. He has potential, loads of it, but I don't see him miles ahead of some other youngsters (Götze for example).
Plus, it's almost impossible for him to fully live up the hype. He might need some time to adapt to Barca, but once he does, he will be great. Not a legend, or an ATG, but great.