It is easy to see how many followers someone has on twitter and how many likes on Facebook. Is there any way of finding out how many people have signed up for an RSS feed?
No easy way. RSS feed content is served up like a web page by a web server, so it is as easy as looking up web site stats for a page and identifying unique visitors. So, that all depends on how easy it is to access the logs, whether you host feed yourself etc, and if you have raw access to the log data, how familiar you are with interpreting and collating it.
You can if you use Feedburner. WordPress will also tell you how many subscribers you have, whether it is followers or via email. But I haven't checked mine in a while and can't remember exactly what it can track. I do remember that there are some things I wish it could track that it doesn't
Before they got rid of it, my recollection is that you used to be able to tell how many subscribers there were to a particular feed on Google Reader too, although that would only have been the number who were using Google Reader to subscribe and didn't take into account the many other ways to subscribe.