Overwatch 2 shifted to a 5v5 format as opposed to the 6v6 setup in Overwatch, a change which was arguably the biggest one to adapt to. Many preferred the previous team sizes which allowed for 12 players total in a match, but as it stands, there's no option in the game's main multiplayer settings to return to the time of 6v6. But as some players have discovered, there is at least an option to play a 6v6 match if you mess around with the settings in the custom game options.
The discovery of the 6v6 matches in custom games may be something players have come across already since Overwatch 2's rocky launch, but the find is gaining more attention now as players post about it online. By going to the Custom Game option and creating your own custom match, you can then go to the lobby settings menu where you'll see an option for max players on a team. Bump that number up to six and you'll be good to go.
That's as high as it goes, however, because moving it above six players per team prompts players with an error saying the settings must be readjusted. So chaotic modes above 12 players in a match aren't possible, but if you've got 12 players willing to make two teams of six to take part in a custom match, you can do so easily.
Players speculated about whether this was intentional or an oversight on Blizzard's part, but the fact that the settings allow for a max of six people on a team and no higher than that seems like the cap is intentional and known. Others have wondered whether or not the existence of this custom match setting means that Blizzard may entertain the idea of putting a 6v6 playlist in the game at some point, but nothing of that sort has been talked about given how recent Overwatch 2's launch is. Blizzard's hero adjustments and other changes were made with the 5v5 format in mind, so it seems unlikely the developer would be willing to compromise with an alternate PvP playlist anytime soon.