Number of RSS Subscribers?

Not possible, unfortunately! There are ways to estimate this number but if you want to keep close tabs on your audience size RSS doesn't offer possibilities for precise surveillance

What are those ways? I’m not interested in precise surveillance; an estimate would be just fine!

I think you could proxy your feed through another service, like Feedburner and collect basic analytics that way. If you wanted to migrate a feed with existing subscribers, you'd do something like this:

1. create duplicate feed at a distinct URL on your site, e.g. /proxy.rss
2. proxy this feed through Feedburner
3. redirect your old feed on your site, e.g. /feed.rss to the Feedburner URL