We made benchmark tests with XFCE on an Ubuntu x32, Cloud Server v. 4 with xfce4 Desktop i5-3427U CPU @ 1.80GHz, two cores processor 16GB of RAM. These results apply to the machine where sessions are running, so in the case of a multi-node setup, they apply to each NoMachine node and are valid for whichever product which supports virtual desktop sessions.
Remember these are just guidelines, it will depend on what DE you are running, and what applications are running on those desktops.
An XFCE desktop:
Empty desktop - 120MB to 160MB (nothing running)
Firefox - 30MB to 70MB per Tab
YouTube video - 50MB to 140MB/1080p
Open office - about 30MB
xfce4, depending on usage, 200-500MB per virtual desktop.
GNOME you should consider approx. at least 100MB more, and for KDE at least 130MB of RAM more.
Our server's 16GB of ram was able to handle without overload about 50 virtual desktops sessions (approx 300 MB per session)
If you were to use a lighter desktop environment like LXDE you could possibly provide about 65 "normal" sessions.
Flash video uses more memory and CPU, so this will need to be considered during your evaluation and will reduce the total number of functional sessions that you can get on your server.
Note for multi-node environments
In load-balancing, the Enterprise Server will be your entry point to the NoMachine nodes. You can decide to run sessions on the Enterprise Server host or not. If not, all computation takes place only on the nodes, so the Enterprise Server can handle multiple user sessions without degrading performance.
If you have activated the cluster functionalities i.e. you have a primary server (Enterprise Server) and a secondary server in cluster, it is highly recommended that you don't run sessions on the primary server host. That's because in case of failover of the primary server, only sessions running on the NoMachine Nodes can be recovered. Additionally, it's suggested that both servers have the same specifics (same hardware and Operating System).
Note for RDP session running in a virtual desktop
The following benchmark test applies to this scenario:
NoMachine Enterprise Client (any) -> Linux server (gateway, with Enterprise Server installed) -> Windows server (RDP Server installed)
NoMachine Enterprise Client on Windows 8.1
NoMachine Enterprise Server on Debian 7, VirtualMachine, 1GB ram, one core i3
RDP Server on Windows 7 Enterprise
RDP Server on Windows XP Professional, SP3
RAM required on the NoMachine Linux server host for the RDP session is from 70MB to 100MB.