I want a mirror server but how to manage the licenses?

You can purchase additional licenses of cPanel/WHM at: http://www.cPanel.net/store

Alternatively, you could manually swap the IP addresses of the primary and backup servers when your primary server goes offline and only need 1 license. The full server backups generated by WHM are .tar files so you can simply use command-line utilities supplied by your OS to restore those files.

However, over the long-term it may be more efficient to rsync the contents of the servers than constantly generating/restoring backups. Feel welcome to submit a support ticket so our technical analysts can provide you with a list of paths and files that would need synchronization.