A reliable subscriber count is not possible, unless you require registration to access your RSS feeds.
For example, my site is being autopolled by 5 other sites using auto aggregators, including 2 or 3 devices of mine, including Google Reader. I count as 1 subscriber, yet if I counted subscribers by how many accesses I get, I now count as 9 or 10 subscribers. ( Until recently even the Googlebot search indexer would show up here )
What may be a better approach is to make your RSS feed items links to your posts, rather than containing your posts in their entirety. You would annoy a few people, but you’d be able to track users arriving via Google Analytics, giving a much better view of how many people use your feeds, and removing the automated components.