Are there no such hack alrady made, that shows how many users that was online at the most?

There is. It is part of the IIP. Look in the Forum Desig Integration

Thanks for feedback....though I would like just the most users ever part integrated in a normal threads forum, not the whole IIP index package...

Here is an older hack for that, I don't know how difficult it would be to update it for 6.x.
https://www.ubbdev.com/threads/php/showflat.php?Cat=&Board=modsuggestions&Number=51407