Entirely based on ViViKo's original "MorePlayers" mod.
For your sake, back up your saves before trying anything. This works as of October 2025, but who knows what'll change in the future.
Save files can usually be found (on Windows) by navigating to %userprofile%\AppData\LocalLow\Alcedo Games\Old Market Simulator\ and are stored in the slot folders.
Download and install your favourite mod manager (r2modman, Gale, etc.) and hopefully it'll do the job for ya?
Grab yourself BepInExPack, pop the DLL from this mod into your plugins folder (run the game once if it doesn't exist), and hopefully nothing crashes.
At some point, the devs at Alcedo Games made a change that involved the player limit no longer being defined in GameManager.OnServerStarted(). That's where ViViKo's original mod was patching the player limit from 4 to 16. Now it's chilling over in SteamLobbyManager.CreateLobby(), so the Harmony Transpiler needed to be pointed at the new function.
I saw a few other people asking for an increased player limit, so I figured I may as well upload this instead of being lazy.