Do you mean the number of comments on a blog post, or the number of comments in a message board attached to an article page?
If it's for the number of messages on a message board, I'm currently using the following to display the number of comments within a preview transformation for news items. It should probably also work for articles, given you customize the code for your environment and database.
This is included in the selected columns:
(SELECT COUNT(*) FROM intranet.dbo.Board_Message WHERE Board_Message.MessageBoardID IN (SELECT BoardID FROM intranet.dbo.Board_Board WHERE BoardDocumentID = DocumentID)) AS CommentCount
and this is in the transformation to display the number:
<a href='<%# GetDocumentUrl() + "#comments" %>'><%# Eval("CommentCount") %> Comments</a>
If you're asking for comments on a blog post:
<a href="<%# GetDocumentUrl() %>#comments"><%# BlogFunctions.GetBlogCommentsCount(Eval("DocumentID"), Eval("NodeAliasPath")) %> comments</a>