Once there are 20 teams, go to 19 games. Play each other once. 190 games isn't a big reduction from the 198 of 22 games.
22 teams, 21 rounds (or two divisions with prom/rel, 12 teams at 22 games and 10 teams at 18 games).
22 teams, 21 rounds (or two divisions with prom/rel, 12 teams at 22 games and 10 teams at 18 games).