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.

danikoo

NEW: 
1. added – option to choose multiple post types, example: ?call_custom_simple_rss=1&csrp_post_type=page,post 
2. added – option to choose multiple post status, example: ?call_custom_simple_rss=1&csrp_post_status=publish,draft 
3. added – better documentation (tutorial).
A plugin to create a your own Custom Simple RSS Feed 
according to parameters you choose!
*** the best solution for using MailChimp RSS campaigns ***
in simple words: 
Ever wanted an rss feed for just a specific Author and specific Category?
Or Even an rss feed for a specific Custom Field ???
Well… now you got it !
The plugin does not alter your default wordpress feeds – it enables you to display feeds on the fly via specific url with pre defined url query parameters.
for example:
display only 5 items from specific category order by name descending:
www.yordomain.com/?call_custom_simple_rss=1&csrp_posts_per_page=5 
&csrp_orderby=name&csrp_order=DESC&csrp_cat=4
Filter items by:
<ul>
<li>category id</li>
<li>post type</li>
<li>post status</li>
<li>tag</li>
<li>range of dates</li>
<li>and even meta keys and values!</li>
<li>NEW! filter by custom taxonomy</li>
</ul>
Order by:
<ul>
<li>name</li>
<li>date</li>
<li>author</li>
<li>ID</li>
<li>etc</li>
</ul>
More Features:
<ul>
<li>number of items to return</li>
<li>pagination </li>
<li>show post thumbnail or not?</li>
<li>set post thumbnails size to display?</li>
<li>show post custom fields (espically usefull if your using your rss as an affliate feed)</li>
<li>filter by date published or modified</li>
</ul>

Custom Simple Rss

A plugin to create your own Custom Simple RSS Feed according to parameters you choose

Feed Comments Number vs Custom Simple Rss