It's difficolt to say....
I think the best way to is to see how the stadium is full and both Inter and Milan make around 80000 fans go to see them when they play in San Siro. But I think Milan has more fans around Italy than Inter. Roma and Lazio make Olimpico full also... I think they have nearly the same number of fans.
Juventus have not so many fans in Turin and that's why they often play in a demi-empty stadium, but they have a lot of fans all around Italy, also in Milan and Rome. Overall they are the Italian team with more fans!