For example:
I am trying to make a private Multi-player FFA game limited to 16 players. In tournaments you can set the number of participants and invite however many people you want. I'm not seeing that same option for the FFA game unless you use the "open seat" option, which kind of defeats the purpose of the private game. When I invite directly it shows everyone invited as being in the game without the option to decline. I feel like I'm missing something but I don't know what.

"When I invite directly it shows everyone invited as being in the game without the option to decline."

I am pretty sure they are still able to decline. So this is the way to go.

You can limit the number of players in a game by inviting less players and/or open seats.

If there are 4 seats in the game, at most 4 players can join. If you add 40 seats, at most 40 players can join the game.

If you invite someone to your game directly, they are not auto-joined. They can decline your game.

Just ask people to not join if N people have already joined.