I have a Windows Server 2008 R2 DC and inside this I have 10 Windows Server 2008 Standard Edition SP2. If I understand correctly, I can run as many OS as possible, but how do I activate these OSEs?
Yes, you can. Whether it is legal is another question (as in: standard edition does not allow 10 vms without additional licenses).
Normally. WAK or KMS. Like any windows instance.