This One Line of code will list the count of VMs on each datastore in the Datastore Cluster
Just edit the datastore cluster name below:
foreach($a in (get-datastorecluster MyDataStoreCluster | get-datastore)){write-host $a ($a | get-vm).count}
List will look like this:
Datastore-1 12
Datastore-2 10
Datastore-3 9
Datastore-4 1
Datastore-5 42