First thing you need to do is go to Settings » Reading page in your WordPress menu.
Next, you need to change the value next to ‘Syndication feeds show the most recent’ option. Simply enter the number of posts you want to be shown in RSS feed.
Below this option you will also see the option to show either the full text or summary of your posts in RSS feeds. We recommend using summary because it reduces the feed size and increases your pageview. After that you need to click on the save changes button to store your settings.
That’s all, you have successfully limited the number of posts displayed in RSS feed of your site.