Well, head to head is going to be one of the biggest factors. So having more than 6 teams means you can't do simply OOC with everyone.
And i used the wrong word, I guess it's not really a tournament. But at the same time, you need definitive scoring and H2H matchups, so having too many would hinder this I believe.
Am I just yammering and not making sense? Im not sure I am being coherent.