On the Global Settings tab, you can specify maximum concurrent backup tasks per storage and max concurrent recovery tasks per node and max recovery speed per VM, as shown below:

Backup Tasks: Specify how many backup tasks can be implemented concurrently, according to the bandwidth and performance of the backup datastore.
Backup Archiving: Specify how many backup archiving tasks can be implemented concurrently, according to the bandwidth and performance of the backup datastore.
Backup Recovery: Specify the maximum concurrent recovery tasks supported by a node according to system performance.

Backup Merging: Specify how many merging task can be implemented concurrently, according to system performance.