Is it possible to fetch a list of comments for a given node by using the rule After saving a new comment? After this I would like to change a taxonomy term for a certain comment having a certain term.
If your rule reacts on event
"After saving a new comment",
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: