May as quick sanity or for all cases. 
As pre-creating custom VMs filled with data and disks, to fit the test cases would be done once rather than per each test case. Speeding up execution on the cost of wasting storage space per the dedicated VMs. 

Think which test cases/source VMs can be combined in same migration plan.  
Again a few migration plans with custom VMs will finish say 80% of the tests. 

The issue is if a single VM fails to migrate we need to investigate which testcase/VM failed translate a failing VM from a plan of a few VMs to the actual failing issue.