Every private comment an AGENT leaves on a Ticket is counted as ONE private comment.
This is irrespective of the number of tickets. It's just a simple count of how many private comments an agent has submitted during a given period of time.
That way, if an agent leaves 10 private comments on 1 ticket, their total private comments will be 10. If they leave 10 private comments on 10 different tickets, their total is 10 as well.