1. How much concurrent user sessions are supported on a single server without any performance impact. ie., they will be hitting the roundcubemail interface. Also highlight on the server hardware configuration required.
Default max concurrent IMAP process is 500. You can increase it in dovecot config file.
2. For redundancy, can we have multiple iRedMail servers acting in load balancing mode.
It depends on your cluster solution.
3. In the above case, will we be able to split users mailboxes among them.
Excuse me, what do you mean "split users mailboxes"?
4. When we have multiple servers, is there any way to replicate the the ldap and choose the correct mailbox server were their mailbox resides.
Default iRedMail setting doesn't support this, you have to adjust some settings.
5. When LDAP is used for authentication, what is mysql used for. ie., what information gets stored in them.
Roundcube/Cluebringer need a SQL database to store their data. OpenLDAP is used to store mail accounts.
6. If mysql is used for storing information, how it can be replicated to the other server.
MySQL replication.
7. Can we split the various functionality like webUI, postfix+imap, mailbox so that we will be able to make load balancing easier & managable.
Sure.
8. Provide us on the backup restore strategy that can be used for the above setup.
*) For mail accounts, backup LDAP/SQL database.
*) For mailboxes, it depends on how you backup it. You can use rsync or similar tools.