Do I Calculate Needed Licenses By Virtual or Physical Server Counts?
You will need a license for each virtual machine running a Windows Server Operating System with Remote Desktop Services or Citrix deployed you wish to monitor; not just one license per physical server, unless each physical server is not virtualized with other virtual machines present (e.g. Hyper-V / VMWare).
In the less common scenario where you wish to monitor inbound RDS connections / Citrix XenDesktop connections to either physical Windows workstations (e.g. Windows 7, Windows 8, Windows 10, etc) or virtual desktops running a Windows workstation OS, you will need to also pickup an equivalent number of VDI/workstation licenses.
Note 1: Workstation/VDI licenses are not required for the client devices (tablets, phones, Windows clients running Citrix Receiver or the Microsoft Remote Desktop Client (mstsc.exe)) connecting in to Windows servers running Citrix/RDS roles.
Note 2: Licenses are not required for each named user or concurrent user connecting into an RDS collection/Citrix farm. Even if you have a high user to RDS/Remote App/Citrix server density (e.g. over 40 users per session host), you still only need one server license per each virtual machine running the RDSH/Citrix session host role.
In terms of where you need to install our software, you can install it either on a physical server or a virtual machine. You will not need to install the main software components on each VM you are monitoring provided that every VM is a member of the same domain. Depending on what you want to monitor, you may need to install the Agent Components on each monitored VM.