Resolution is outdated. Unless resolution has no effect on maximum player tab count.
Edit: I thought from pic you were trying to throw more on there. I wish chat could be hidden period. Maybe it's my own OCD but I don't like it open unless I'm actively using it.
Might as well post this issue into suggestion forums though... I agree with the scrollable chat tabs.
In the meantime, you can also clean it up by looking up "ChatChannels" and "ChatLastChannel" inside your osu.<username>.cfg file and remove all the values.