The other day my YT started showing only 4 videos per line on a 4k screen set to 2560x1440, with even more space available if I wanted. I jumped onto Reddit and found this. It didn't work at all and gave up. But today I decided to look again and saw that Pedro released another version and recognized that YT had changed something fundamental. With that it is all fixed. I set it to 6 icons per line just like it was before YT "fixed" things. I'll have to explore the other settings too, there were too many to get through.