To see the number of subscribers count for any feed click on Add Subscription button in your google reader and search with the name of a blog or keyword and it shows the subscriber count next to each feed.
These numbers represents the subscribers via google reader, iGoogle and Orkut. These numbers vary with feedburner subscriber count, Google Reader blog explains about these differences.
I checked my blog subscriber count in feedburner and it shows 295 subscribers today and this is slightly higher than what feed discovery shows above.
These numbers may not speak about the total number of subscribers a blog has but you can estimate the total subscriber count. New blogs will have higher google reader subscribers and the old blogs have higher bloglines subscribers.
Update: Mashable has an interesting post on these numbers