Please refer to the Exchange Plugin User's Guide for more details on this:
Maximum Number of Parallel Streams – To enable the fastest backups possible, the Plug-in for Exchange uses parallel streams to back up the Storage Groups/Mailbox Databases simultaneously. This option allows you to specify the maximum number of parallel streams to use
during a backup. The maximum number of parallel streams must meet both of the following conditions:
• Less than or equal to (<=) the number of drives for the backup devices specified on the
Target tab.
• Less than Maximum number of Storage Groups/Mailbox Databases that can be created.
The following table details the maximum number of Storage Groups/Mailbox Databases allowed by each Exchange Server version/edition:
*For example, if you are targeting a Virtual Tape Library configured with 2 drives and you have 20 Storage Groups, the Maximum Number of Parallel Streams should be 2 or less.
Maximum supported
Exchange Server 2003
-Standard 1 Storage Group
-Enterprise 4 Storage Groups
Exchange Server 2007
-Standard 5 Storage Groups
-Enterprise 50 Storage Groups
Exchange Server 2010/2013
-Standard 5 Mailbox Databases
-Enterprise 100 Mailbox Databases
NOTE: In an Exchange 2010 or later DAG deployment, the number of parallel streams is allocated on a per-server basis; therefore, the number applies to each server included in the DAG. For example, if your DAG includes three Exchange Servers and you select two streams for backup, it can create up to six simultaneous streams assuming that there are at least two databases on each server that need to be backed up. If there are fewer databases, the streams are not created. If there are fewer drives available, the streams wait until the next drive is available.
https://support.quest.com/search?k=netvault+Exchange+Plugin+User+guide
