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.

scribu

This plugin adds feed auto-discovery links to any page type:
<ul>
<li>Category page</li>
<li>Tag page</li>
<li>Search page</li>
<li>Author page</li>
<li>Comments feed for single articles and pages</li>
</ul>
It also has a template tag that you can use in your theme.
Links: <a href="http://scribu.net/wordpress/extra-feed-links" rel="nofollow ugc">Plugin News</a> | <a href="http://scribu.net" rel="nofollow ugc">Author’s Site</a>

Extra Feed Links

Adds extra feed auto-discovery links to various page types (categories, tags, search results etc.)

Feed Comments Number vs Extra Feed Links