Hmm, I guess it's pretty hard to estimate the number of all fans worldwide but if there really has been done a some kind of research about that I'd like to know how has it been done and what were the results.
In Singapore I seen a lot of ppl wearing these jerseys from these 2 teams, there r already quite a number of ppl wearing Man U + Real jerseys from my course ALONE. I think I would have bought a Juve jersey too if I could afford it. $95 for a shirt isn't cheap.
People who don't know football, support big teams, with big names. i.e. Teams like Real Madrid. The other type of football "fans", follow teams that are famous in their respective leagues, i.e teams like Man United and Milan. Then there are the loyal fans, who support teams they truly love, like the fans of Juventus