Backup Exec Licenses

Dears,

I need to know how many Applications and Database license need to protect:

* Two mailbox cluster (Exchange server 2007 CCR)

* Two Domain Controllers (Windows server 2008)

I want to confirm that I will need 4 Licenses, for each node on the two clusters one license. Am I correct ? or I need only 2 licenses. if there any Symantec document describe the cluster licenses , in both case Active/Active and Active/ Passive.

Thanks all for your cooperations

Layth