Is there a plugin out there that makes the slots of the server one more than the actual player count? Like what Mineplex is doing:
Yes, but it would take a custom plugin to do. Also, the maxPlayers you see is not the server's real maxPlayers. The maxPlayers you see in the server.properties is the maximum amount of players the server will accept. As for the max players you see in the Multiplayer List, that can be changed by a plugin.
Possible plugin choices, Up-to-date plugins, FakePlayersOnline Abandoned old plugins, (still may work dont see why this method would be changed/deprecated) Enhanced Max Players MaxPlayers
