RSS Statistics
You can not have an RSS feed without having some way to measure how many subscribers you have. Here is pretty much your only option. Somehow, this seems to be the onlye one available 
Feedburner
Not much has changed since it was swallowed by Google. The no sweat option, although numbers can vary widely, taken over a few days, the numbers look pretty good. The drawback? You need to have people subscribe via the feedburner URL.