TNManager.playerID is incremented internally when a new player *connects* to the server.
The order players join depends on when each client hits TNManager.JoinChannel and in turn fires the onPlayerJoin event. Keep in mind that onPlayerJoin does not fire for the client/host, only other players. Use onJoinChannel to detect when the client/host joins the channel.
How you assign each player depends on your game logic. For example, if all 6 slots fill up and then slot 4 disconnects - either shift players around so the next join is player 6 or simply replace slot 4.