You should check your Configuration Maximums guide for the version of ESX that you are using and do the math based on your cluster config and workload:
For Example, vSphere 5.1:
Concurrent Storage vMotion operations per datastore 8
Concurrent Storage vMotion operations per host 2
http://www.vmware.com/pdf/vsphere5/r51/vsphere-51-configuration-maximums.pdf