Player Limit in a Minecraft server
It's the result of using bungeecord which is a Spigot plugin. If you would want to achieve the same result, just put this in your bungee configuration file:
listeners:
- max_players: 1
If you don't use bungeecord and would want to do it anyway (and you use essentials), set the player slots to 1 and give everyone the permission
essentials.joinfullserver