I don't think it is out of the question for him to win 6 majors before his career is over. Outside chance at winning the grandslam but I think he may have trouble winning the British. Definitely think there is a U.S. open in his future.
I think these things are always overestimated - I tend to think 1 or 2 more max. He may not win any more at all.
He's a good player no doubt but there are players in this era that are head and shoulders abover him and young players like Scott who are likely to break through.
In my estimation there are 2 players guaranteed to win a few more majors then the rest can win if they have a good week and the elite 2 are slightly off.
4 more if he keeps in shape.......he's at the peek now for golf and the youngens haven't really lit the golfing world on fire aka Sergio,Howell,Scott,etc. Would say 2 more before he hits 40 yrs old and a few after that.