All correct there - the 3, 5 or 10 nodes is for monthly or yearly (any FairSaaS license). If you purchased the "standard" FairSaaS that is for 1 Workstation and 3 render nodes, so to use more for your farm you'd need to change that to the 5 or 10 option. You can always contact support to discuss the best licenses for your needs, tell them how many workstations and render nodes you have and what your current license is, and they can advise and swap you over to whatever is needed.
On GPU, you don't even need one for a pure render node, just onboard graphics will be fine (only exception is if you are submitting a single image to a render node via backburner and you want to use the NVIDIA denoisier, then you'd need a compatible NVIDIA card in that render node)
How it works is simple - run Max and the Licensing Server on your master machine, then Corona DrServer on each render node, and you are done. You will need 3ds Max installed on the render nodes, but it doesn't have to be activated and licensed, just installed (which is the legitimate way Autodesk expect you to use Backburner and network rendering). You will need the same version of Max, same version of Corona, same version of all plugins, installed on all machines.
