Hyper-V itself is free to load up as much as you want (as far as VM's go). However the VM's themselves need to be licensed. If it's Linux then there is naturally no license required. However for Windows VM's there are several ways to go.
Using Server 08 R2 as the reference per hypervisor.
Standard License = 1 Physical and 1 Virtual
Enterprise License = 1 Physical and 4 Virtual
Datacenter License = 1 Physical and Unlimited Virtual
Now it has been a while since I've spoken to a rep, these things might be off or have changed. But that should give you a basic idea of how things work. Hyper-V itself is included standard and there are no additional licenses for Hyper-V by itself, it's a standard feature of Windows Server.
As i know, upon we create virtual and install Window OS into Virtual machine, the window still need to go through the license activation. Upon we activate it, it will count as 1 license or no?
It depends on the version you have, it will count as an "activation." Weather or not you have the correct license is up to you to keep track of. Obviously if you don't have the proper licensing they might stop allowing activations on a given key.
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I am not really sure. I am using Window server 2008 Data Center R2 with MAK activation key.
Then you should be fine with all the VM's you put under that single hypervisor. i.e. if you want to put more VM's on another hypervisor (server) you'll need to pay for another DC license. Also I'm not sure if that's licensed by socket as well, best to talk with a MS license specialist (or three) sometimes you'll get different answers; better to be safe and not worry.