This plugin will display an image showing the number of comments on a post at the bottom of each feed item (RSS|Atom|etc) that links to the comments section of that post. 
You are able to choose which colors to use for the background of the image and the color of the text as well. 
The reason you would want to use an image instead of plain text is because RSS feeds are generally only updated if the build-date or modification-date components are changed, which does not happen if someone leaves a new comment. This will allow the comment number to update dynamically, regardless of when it’s been first viewed by a user.
You’re able to set the colors of the text and the background in the standard 6 character hex format. 
You may also supply formats for what will show if there are zero comments on your post or more. 
I just added the option to include the image link in the feed excerpts as well. (called by <code>the_excerpt_rss</code> function)
Two Font files are included with the plugin, Arial and SenateBold. You can set which of the two you would like to use, or optionally upload your own .ttf font file to use to render the text on the image.
This requires the GD Image library extension in PHP to function. To check whether or not your server has this use the <code><?php php_info(); ?></code> function and it should be listed.

Feed Comments Number

Add an image that displays the number of comments to your feed items.

C. Johnson

<ul>
<li>Author: <a href="http://feedback.radgeek.com/contact" rel="nofollow ugc">C. Johnson</a></li>
<li>Project URI: <a href="http://feedwordpress.radgeek.com/" rel="nofollow ugc">http://feedwordpress.radgeek.com/</a></li>
<li>License: GPL 2. See License below for copyright jots and tittles.</li>
</ul>
FeedWordPress is an Atom/RSS aggregator for WordPress. It syndicates content from feeds that you choose into your WordPress weblog, and then the content it syndicates appears as a series of special posts in your WordPress posts database. If you syndicate several feeds then you can use WordPress’s posts database and templating engine as the back-end of an aggregation (“planet”) website. It was developed, originally, as a utility/hobby project, because I needed a more flexible replacement for <a href="https://web.archive.org/web/20051029095046/http://www.planetplanet.org/" rel="nofollow ugc">Planet</a> for aggregator sites that I administered.
FeedWordPress is designed with flexibility, ease of use, and ease of configuration in mind. You’ll need a working installation of WordPress (version <a href="https://codex.wordpress.org/Version_4.5" rel="nofollow ugc">4.5</a> or later), and it helps to have SFTP or FTP access to your web host. The ability to create cron jobs on your web host is helpful but not required.
<h3>Using and Customizing FeedWordPress</h3>
FeedWordPress has many options which can be accessed through the WordPress Dashboard, and a lot of functionality accessible programmatically through WordPress templates or plugins. For further documentation of the ins and outs, see the documentation at the <a href="http://feedwordpress.radgeek.com/" rel="nofollow ugc">FeedWordPress project homepage</a>.
<h3>License</h3>
The FeedWordPress plugin is copyright © 2005-2021 by Charles Johnson. It uses code derived or translated from:
<ul>
<li><a href="http://laughingmeme.org/archives/002203.html" rel="nofollow ugc">wp-rss-aggregate.php</a> by <a href="kellan@protest.net" rel="nofollow ugc">Kellan Elliot-McCrea</a></li>
<li><a href="http://www.simplepie.org/" rel="nofollow ugc">SimplePie</a> feed parser by Ryan Parman, Geoffrey Sneddon, Ryan McCue, et al.</li>
<li><a href="http://magpierss.sourceforge.net/" rel="nofollow ugc">MagpieRSS</a> feed parser by <a href="kellan@protest.net" rel="nofollow ugc">Kellan Elliot-McCrea</a></li>
<li><a href="http://diveintomark.org/projects/feed_finder/" rel="nofollow ugc">Ultra-Liberal Feed Finder</a> by <a href="mark@diveintomark.org" rel="nofollow ugc">Mark Pilgrim</a></li>
<li><a href="https://wordpress.org/" rel="ugc">WordPress Blog Tool and Publishing Platform</a></li>
</ul>
according to the terms of the <a href="http://www.gnu.org/copyleft/gpl.html" rel="nofollow ugc">GNU General Public License</a>.
This program is free software; you can redistribute it and/or modify it under the terms of the <a href="http://www.gnu.org/copyleft/gpl.html" rel="nofollow ugc">GNU General Public License</a> as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version.
This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.

FeedWordPress

I've been using FWP for years. I used to have several sites that relied on it for aggregating ebay stuff. However, on shared hosting, and with MANY syndicated sources, my sites often got banned for hogging the server.
The "problem" is that FWP is so excellent at what it does, I got carried away with the number of sources. 
I've been using a different aggregator for a while, but it doesn't have the flexibility of FWP: setting Category, Tags and custom fields per feed is awesome.  
Hence, I've come back to using FWP, but not using the scheduling features, and taking it easy on the number of sources.  
If you are NOT on a shared server, just ignore my comments and "fill yer boots". 

Excellent! Be careful on shared hosting, though..

I struggled with multiple RSS aggregation plugins, all of which were paid subscription plugins. I had already paid for one and after hours, I could not make it import enclosures to PowerPress. FeedWordpress took about 5 minutes to set up and worked perfectly. It also had the feature to all for reimportation of items which the paid plugin lacked. This free plugin is in every way better than the commercial alternatives.

Simple Install and Works Great

Have used FeedWP for multiple projects over the decades, for monitoring news on specific topics. Works flawlessly, and offers many valuable options. A well-designed workhorse.

Excellent and dependable.

FeedWordPress syndicates content from feeds you choose into your WordPress weblog.

Feed Comments Number vs FeedWordPress