After saving a new comment How to fetch all comments for a given node?
If your rule reacts on event "After saving a new comment",
add a Loop in actions section and select comment:node:comments as a selector.

The loop will go through an array of comments from commented node. There you can specify additional actions/conditions that you wish to perform on each comment.

PS: You will run in the issue of not being able to modify comment fields via Rules


  "Unable to modify data "field-taxonomy-term": Entity property
  comments doesn't support writing."


Here is more information about the issue:


"The selected data property doesn't support writing." when Action: Set a data value is applied to a taxonomy field:tid 
How to fix entity property X doesn&#39;t support writing, when setting content type field values?