It means, I have to enter same key in all VMs so Do I have to install datacenter edition OS on all VMs also ?
What if I already install windows standard OS so do I need to upgrade OS to datacenter edition and always install VMs with datacenter OS ?
AFAIK Microsoft grants the right to use lower editions as well as lower versions. Assuming that you've purchased the licenses from MS (rather than OEM/ROK), you should be able to get the required installation images, and keys from your Microsoft licensing portal.
André