Currently in order to calculate these numbers you’d need access to the ClassicPress server logs, which isn’t something we can share publicly. However, we’re working on an automated process to publish a graph of active sites over time.
Our general methodology is that we count a site as active for around 7-10 days after it checks in with our update server, because many sites don’t check in very often due to low traffic levels. I think this is a good compromise between counting sites that have really installed ClassicPress and not counting sites that aren’t active anymore.
As Wade says “around 1500 active sites” is a good estimate for now.