Wow! What an original thread topic! But you are forgiven since you have Schalken in your avatar!
I think he'll end with 12 or 13 slams, though I hope he'll surpass Sampras' record 14, leaving no doubt as to who is the greatest ever.
I don't think he will reach 14 grand slams, he might, but I think the standard will be changed. The players these days are just so much more talented and more physically able then the players were during Sampras's time, just look at 19 year old Nadal for example! I think if Federer keeps winning and wins an assortment of titles, including the FO which will be very tough, he could still be considered the greatest of all time!