16 is a kinda traditional number for hosting a final event of the year. You can make 4 groups per 4 teams, then 2 best from each group advance to double elim playoffs etc...
If Riot make Champions 18 or 20 teams the tournament format won't be so smooth like I mentioned before.
My solution is to reduce SEA slots from 2 to 1. Zero viewers, zero competition, zero skills yet these bots will rob some decent NA or EMEA team from attending a tourney.