Change maximal parallel backups (Exchange)

We have one DPM 2012 SP1 (CU3) Server, more than 4 Exchange Server (2010) with a lot of Exchange Mailbox DBs and one Protection Group for the Exchange Backups.

Is it possible, to change the max. allowed parallel backups (for Exchange Mailbox DBs)?
(Express-Full and Synchronisation)

I only found one registry key, but it´s for backup Hyper-V guests.
HKLM\SOFTWARE\Microsoft\Microsoft Data Protection Manager\2.0\Configuration