how to increase # of backup server action, seems for now the limit is 3

I have a few virtual server on backup schedule, it seems sometimes I had this message, so once a while the backup fail

There are currently 3 backups running, and the system-wide limit is 3

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here is one of the fail backup this evening

Backup failed! See the progress output above for the reason why. Total backup time was 00 minutes, 00 seconds.

Virtual servers that failed : food.orderlastminute.com Sent by Virtualmin at: https://ops.orderlastminute.com:10000

There are currently 3 backups running, and the system-wide limit is 3

Howdy -- thanks for contacting us!

You can tweak that option by going into System Settings -> Virtualmin Config -> Backup and Restore, and there set the "Maximum concurrent backups" option to the value you want.