A forum can contain multiple posts. When a student clicks on a forum topic, they see all posts. Consequently, moodle doesn't know (nor does any computer system) if the student viewed every post. Forums do have a log file, and you might find some information in it. I do not know of any report that tells how many "clicks" a forum had.
If you are really interested in student activity, consider forcing every student to reply to every post. Then use "Ratings" as a way of trying to calculate how many posts a student read.