Deleted Comment
It rather depends on the amount of content the RSS _author_ includes in the RSS feed. There's nothing in the RSS/Atom protocol that prevents you from reading the entire article, but some website creators decide to truncate the feed content.
My RSS reader of choice, InoReader, has the option to download the original website which solves the problem. However, I have over 200 feeds and it's rare to find one without the entire content being included.
Some RSS readers are quite aggressive with their polling time. At the same time, a single request may serve thousands of users. However, you can check your logs and get the actual number of subscribers, at least for popular RSS readers: https://darekkay.com/blog/rss-subscriber-count/
[1] https://darekkay.com/static-marks/