It is possible he may need zero since he is only 23 and has 7-8 more years of tennis in him where he could win 3-4 slams a year. But what do you think Nole needs in 2012 to stay in contention for the greatest ever player and the most slams in history?
What kind of backwards logic is this? So he doesn't win any more slams this year, which either means he gets hurt, or his level drops off considerably, or Rafa or Murray improve a lot, or someone better comes along. Either way, how in the world would be he winning 3-4 slams in years after that? What happens to those people that beat him at RG, Wimby and USO?
Anyway, he's nearly 25, not 23. With his style of play, even if he wins CYGS this year, he would still max out at around 10-12 slams, still a HUGE achievement, and would definitely put him among the greatest that have ever played this game.
But one thing at a time. Let's first see if he can win RG (not the first favourite there, IMO) and get the Nole-slam.