SQL Server 2012 Licensing Model

Team,

I need information on SQL Server 2012 Enterprise edition Licensing on individual VM servers.

Ex: I have 3 Vm with below configurations - 

1. 2 Cores(CPU's)

2. 5 Cores(CPU)

3. 8 Cores(CPU)

Kindly request you to elaborate me the number of Licenses I need to purchase for the above configured VM's.

Also if I purchase for 1 VM, can i install multiple instances and use?

Please suggest.

Thanks for your time.

Suman G