When we want to write a comment we see this message: "enter at least $15$ characters"
However it is not an accurate way of measuring the characters used in comments when we are in discussion with other users. For example consider I am in discussion with a user which him/her username has only $1$ character so when I tag that in the comment I spent $2$ characters and I should use $13$ more characters to be able to send the comment. On the other hand consider another user which him/her username contain $10$ characters (or more) in this situation after tagging, $4$ more characters are left to write.
This is a bad situation. If users comment should have a certain length then it shouldn't be changed according to length of a username which is tagged. because it is not a fair situation for all the users on the site (therefore I conclude the number of characters I can use in a comment when I tagged a user is completely random rather than being a certain number ($15$) ).
Below this post We had a discussion about adding characters In edits. to avoid any confusion I would add: this post is about comments not edits or something else.