I know we can add in posts loop using in index.php and in single.php by filtering the content but I don't know how to add after certain comments. For example, if a post have comments, after 30 comments, I want to display a under which I want to put an advertisement. Then after 60 comments, I want to have second and so on. I am not sure if this can be done via plugins but I am refraining myself to use plugins to reduce load on the site.

Please help!