Use the UserCommentsCount snippet do do this.
You need to use the paramaters available in the UserCommentsCount snippet. Don’t forget that the document that is being parsed by Ditto has the id [+id+].
Quote from: rocccio at Aug 31, 2006, 03:36 AM
How do I insert the id into the snippet call? I’m a bit lost.... 
[[UserCommentsCount? &docid=`[+id+]`]]
See
http://modxcms.com/+UserCommentsCount" target="_blank" rel="nofollow">this and
this.
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