More than cuncurrent backups, I'd look at cuncurrent snapshots open on the same datastore, mainly because, regardless the lower performance created by VMs running over snapshot, multiple open snapshot means more space consumption on the same datastore, and more IO needed to consolidate them. For this reason we have a default limit of 4 open snapshot per datastore. Depending on the performance of your environment, you may decide to increase this value using a dedicated registry key.
EMEA Cloud Architect @ Veeam Software
Veeam VMCE #1