Well, since the NVENC allows 2 simultanious encoding sessions (per machine using a consumer GPU). That would require you to have 40 machines with 40 NVENC capable GPU's.
If you have a professional GPU, the encoding sessions are unlimited, but depending on the hardware you should be able to do around 10-12 sessions per professional GPU.
The NVENC session limitation on consumer GPU's is a driver enforced limitation. (also see: https://developer.nvidia.com/nvidia-video-codec-sdk).
I have found the results with NVENC to be quite acceptable. If anyone knows of a way to circumvent the 2 simultanious sessions on a consumer GPU without modding the HW to appear as a professional card, I'm all ears ;)