The free Hypervisor Edition has unfortunately been dropped, so if you want to run ESXi on the mentioned server, you had license 16 cores (the minimum per CPU). The least expensive option in this case would be the Standard edition, which comes as a yearly subscription at ~50$/core.
To be honest, in that special case with a single host, and only two Windows Server VM's, you may consider to install Windows Server on the physical host, and run the two VMs using Hyper-V. If you do not use the physical Windows system for anything else but Hyper-V, a single Windows Server license should be sufficient to run the OS on the hosts as well as the two VMs (see MS licensing for details).
André