Ever resourceful Ionut Alex Chitu has a post that helps you find the number of Google subscribers to any feed.
"Now you can see the number of subscribers by searching for the feed in Google Reader: click on "Add subscription", enter the name of a blog or some parts of your feed's URL and you'll see the number of subscribers next to each (hopefully relevant) result. This number includes everyone who subscribed to a feed using Google, but the subscribers aren't necessarily active."
In response to this, the official Google Reader blog has a post that 'clarifies' the confusion.
"We've made changes (some as recently as today) as to how counts are being calculated. This is probably going to be pretty boring unless you're a feed publisher, but we thought it would be best to explain things a bit. Here are the various numbers you may come across, and what they all mean:
Google subscriber counts: These numbers include subscribers across all Google services, including Reader, iGoogle, and Orkut. You can see them in Reader's feed search results (pictured below) and the Google Webmaster Tools. Additionally, our crawler reports them to the publisher each time we fetch the feed. Reader's feed search was recently showing stale and incomplete data, but as of today (October 15) the numbers should be the same everywhere.
FeedBurner numbers: If you use FeedBurner to manage and track your feed, you will see a subscriber count there that is attributed to "Google Feedfetcher." This number is a sum of all the feeds that you have redirecting to your FeedBurner feed URL. So if http://www.example.com/atom.xml has 3 subscribers, http://www.example.com/rss.xml has 7 subscribers and http://feeds.feedburner.com/Example (where you redirect the other two feeds now) has 12 subscribers, then you will see 3 + 7 + 12 = 22 subscribers reported in the FeedBurner interface.
What this all means if you're a feed publisher is that if you're interested in getting the most comprehensive overview of your subscribers, you should be using a service like FeedBurner or Google Webmaster Tools. On the other hand, if you're a Reader user, we hope you take advantage of the numbers that we now show next to search results, so that you can pick the most appropriate feed to subscribe to."
Thats not all, other experts have lent their own pointers and tips to this:
Gabe Rivera's list of top feeds and popular 2.0 blogs and their Google Reader numbers on TechCrunch
Lee Odden Top SEO Blogs according to Google Reader (weÃ¢â‚¬â„¢re #8)
Matt Cutts adds tips on Google Reader tracking.