If you are migrating a VM from Datastore 1 to datastore 2 irrespective of host.
per datastore you can have maximum 8 storage vmotion concurrently , i.e what below screenshot and provided doc explains:
If you are migrating VM from datastore1 to datastore 2 , per host concurrently you can have 2 SV motion being initiated .
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regards
Gayathri