Addendum: going to hold out for 19.
Long story short, (P-1) where "P" is # of players needs to be divisible by 3. This is so that you're not only playing player x in constructed brawls and player y in pre-made deck brawls and you get to play each player in each kind of brawl at least once
Furthermore, it's the only number "P" that keeps length of Season 1 "L" below 12 weeks which is too long for Season 1 imo. And finally it's the only number "P" that keeps Games/week "G" above 5 which otherwise is too boring.
So 19 it is