License question backup server

==== Required information ====
- iRedMail version:
- Store mail accounts in which backend (LDAP/MySQL/PGSQL):
- Linux/BSD distribution name and version:
- Related log if you're reporting an issue:
====

We are interested in purchasing a lifetime license of iRedAdmin-Pro.

iRedAdmin-Pro would be running on a server that is mirrored by a backup server. If the main server crashes the mirror server would be activated through a cluster IP. Questions: Do we need one or two licenses?

Thanks in advance!