Licensing around running VMWare ESX on a mac mini hosting multiple OS X guests
The OS X EULA does allow for OS X to be virtualized on Apple hardware as both host and guest.  This is why (as you note) VMware Workstation does not support OS X virtualization, but Fusion, ESXi, and vSphere do.  All versions of VMware's apps check to ensure that you are running on Apple hardware and you are running a supported OS (as not all versions of OS X allow virtualization).  

Apple doesn't make it easy to purchase a Volume License.  You either need to buy it directly from Apple (it's not on their website, you have to call them) or through a third-party reseller.  

You are completely correct that the EULAs are unclear about the question of running 3+ OS X VMs on a single OS X host.  This is a legal question, and not a technical one.  For that matter, so is the limitation about which versions of OS X can be virtualized (ex: 10.6 virtualization requires the Server edition, and all VMware applications block you from virtualizing the Standard edition).  If the EULA were clear that you could not virtualize 3+ OS X VMs, or if Apple had said anything to VMware about it, VMware could limit you to 2 OS X guests per OS X host.

If virtualizing OS X is your goal, you should talk with someone who is in Apple Enterprise Sales about what you want to do.  EULA questions really can't be answered in any meaningful way by anyone else.