Login to your wordpress on your website.
Click on READING

You will see "Blog pages show at most" this is how many posts to show on your front page. If you set this to "2" their would be 2 blog posts, and a link for "older entries" below the first 2.

Then there is a box next to "Syndication feeds show the most recent." This is typically set to 10 by default. If you set this to 20 and you have 18 episodes all will appear. If you set this to 20 and you have 30 episodes, only the last 20 will appear.

If you are using Feedburner.com there is a limit to the size of your RSS feed. Experiment and play. When you are first starting out (if you want) you could set this to a high number (100).

If you are using Libsyn, check the settings in your destinations and the Libsyn feed.