License is based on socket basis, but not on per core basis. A socket may contain multiple cores (dual core, quad core etc). However a single physical core can handle a max of 20 logical vCPU's from ESX4 onwards.
Look for below docs for same...
VCP 3&4, MCTS(Hyper-V).
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if you are talking about vmware view, it will go by per client bassis. Which you just need to pay the price per client, and they provide you vsphere 4, vcenter, vmware view 4 and others necessary item for desktop virtualization on VMware.
As ppl pointed out , license is based on socket and not core ... So if you have a dual socket system , you will need 2 licenses ... If you have quad socket system , you will need 4 licenses . The underlying cores does not count .
For example , Dual cpu ( single core or dual core or quad core ) all require 2 licenses . In the same manner Quad cpu ( single core or dual core or quad core ) require 4 licenses .
Hope this helps .
If you are looking for information about
VMware multicore licensing
Check section "How does this policy affect my licensing costs on servers with less than 6 cores per processor?"