Virtual Machines backup used only 10 devices although more than 10 drives were selected on backup specification and concurrency set 30 ( Min=1, Max=30)
Reason for this behaviour is that OB2_VEAGENT_DISK_CONCURRENCY variable limits disk threads to be started in parallel for no more than 10 .
In order to change this behaviour need to modify omnirc file on backup host and set OB2_VEAGENT_DISK_CONCURRENCY to value higher than 10 .
Note that setting OB2_VEAGENT_DISK_CONCURRENCY omnirc variable to low value will decrease backup concurrency and will avoid lack of hardware resources.