Windows Server Licensing : Standard versus Data Center when considering #VMs

Question

Hi Folks ,

My question is about how to correctly license Windows Server ( Std versus DC ) despite #Cores and price . I know these are important but I am really trying to understand the virtualization rules .

To be clear let me give an example considering the customer with one single server and 30x VM´s being 11x VMs running Windows OSE and the others ( 19 ) VMs running Linux Red Hat /SUSE OSE.

Must I license Windows Server Data Center edition because it has unlimited VMs support to both Windows and Linux OSE´s or I can license only 6x Windows Server Standard ( up to 12x VMs ) and consider Red Hat/Suse license rules to the others 19x VM´s ?

I think that if Hyper V is the hypervisor ( the virtualization manager ) I must have Windows Data Center . Am I right , guys ?

Thanks in advance