We are using VMware to virtualize software installations. We are planning to back up our virtual machines with Acronis Backup 12 for VMware. In such case, do we need to purchase a license from Acronis for each virtual machine?

No, Acronis Backup 12 for VMware is licensed on a per host CPU basis. You may use it to back up a Hypervisor Host Server and up to an unlimted number of virtual machines residing on the Hypervisor Host Server.

Source: Acronis Licensing Policy, retrieved 2016-07-12 (https://www.acronis.com/en-en/company/licensing.html)