2 million fedora core 6 installs
We have a statistics page on the Fedora wiki that tracks these metrics, and also talks about what the numbers mean, and where they come from.
In short, for folks who don't know, we track unique IP addresses that connect back to the Fedora Project's repositories searching for updated software. Ultimately it's not much more than just counting up unique IP addresses that show up in our weblogs, but we make it a point to put the numbers out there in public and allow people to analyze them however they like.
One interesting thing to look at is architecture breakdown, now that our sample size is very large:
x86 -- 87.94%
x86_64 -- 11.72%
ppc -- 0.35%
While we're on the topic of metrics, I'd like to point people at Smolt, a project being led by Mike McGrath that is a basic hardware profiler for Fedora.