Gamelift Flexmatch with Minimum Players and Maximum Players to start a Match

I am trying to understand how flexmatch works and I want a simple matchmaking ruleset where matchmaking should start with a maximum of 50 players. But if not enough players are available, I want the queue to process match if atleast 25 players are there. Then from lobby I can do backfill. But I am not able to make this logic in ruleset. Any sample ruleset for 1v1 with minimum and maximum player logic will be highly appreciated.