1. There will be only one message server through out the System.
2. It is used to manage all the dispatchers and identify the least loaded dispatcher and forward the user request to the dispatcher provided logon load balancing is configured in transaction SMLG
Logon Groups SMLG
SMLG > Create
Provide GROUP: For Eg: MARKETING
Now from SAP Logon Screen
Click on Groups > and Provide SID and Message Server.
We can find the Active servers from SM51 and on db click on the Host name to view all the Processes.
You can also get information regarding SAP BASIS