(1) Download this JS file and save it to your modern/ template directory. Open it and edit the two var entries on first two lines to (a) match the backend comment limit number and (b) to match your language.
(2) Open the modern index.html file and search for this codeblock:
Or if it's easier just take the whole modern template with these modifications I just made to test this. Extract it to "modern" and over-write or you can also extract it to a new folder under templates e.g. "modern-tick".