in vSphere 5.0 and 5.1 (http://www.vmware.com/pdf/vsphere5/r50/vsphere-50-configuration-maximums.pdf , http://www.vmware.com/pdf/vsphere5/r51/vsphere-51-configuration-maximums.pdf)
the Maximums -
Concurrent Storage vMotion operations per datastore - 8 Concurrent Storage vMotion operations per host - 2