Virtual Machines backup used only 10 devices although more than 10 drives selected on backup specification and concurrency set to 20 ( Min=1, Max=20) .
The reason for this comes for default variable OB2_VEAGENT_DISK_CONCURRENCY setting on omnirc file on backup host . OB2_VEAGENT_DISK_CONCURRENCY variable limits disk threads to be started in parallel for no more than 10 .
In order to change this behavior need to modify omnirc file on backup host and set OB2_VEAGENT_DISK_CONCURRENCY to value higher value ( higher than 10 ) .
Note that high value for this variable can cause for lack of hardware resources and the whole backup can fail as it depends on the available hardware resources on the machine .