I personally think it is much less.
To give you an idea, I've been a member of two soccer fan clubs in two huge universities here- in Virginia and New York. Both schools had some 40,000+ students each. In both schools I was the ONLY Juventus fan, even among international students. So, that's a rate of 1 to 80,000.
I am still yet to see a single Juventus fan, other than people on the internet.
A lot of people consider themselves soccer fans/followers, even among Americans, but I doubt there are many Juve fans among them.
My conservative estimate is a maximum of 30,000 Juve fans nationwide.