Some sites displays their feed suscribers by using static or annimated graphic image of their FeedBurner feeds, but some do not show due to one or other reason. I too don’t show, simply ‘coz I don’t have much subcriber.
Here's a trick to find out the No of Visitors suscribed to any feed powered by FeedBurner, only if the feedcount has been enabled by the publisher.
All FeedBurner feeds has a url address like feeds.feedburner.com/xyz, you just have to add ~fc/ before xyz [Site Feed name], for e.g. feeds.feedburner.com/~fc/xyz
Note: Replace the xyz with actual feed name.