Exchange Server 2013 Licensing in virtual environment

Hi team,

In a scenario like below, how many Exchange Server 2013 Licenses are required?

• Physical Server 01 (OS Only)
• VM 01 - CAS/MBX
• VM 02 - MBX

• Physical Server 02 (OS Only)
• VM 01 - CAS/MBX
• VM 02 - MBX

Thank You.
Chris!