Okay, according to the results of this survey there are 7 to 12.4 mio. Bronies in the US.
http://herdcensus.com/generalsurvey.shtml
Furthermore, here's a statistic from EqD, back when they celebrated their 200,000,000th visit, apparently there have been over 6,000,000 unique visitors.
http://www.equestriadaily.com/2012/06/200000000-celebration-post.html