There is a NVIDIA VIDEO CODEC SDK APPLICATION NOTE - ENCODER in which we can read the following:
While Kepler and first generation Maxwell GPUs had one NVENC engine, certain variants of the second generation Maxwell GPUs and Pascal Generation GPUs have two/three NVENC engines physically present on the silicon.
So, does anyone know how to determine the number of NVENC engines in a GPU or already know the numbers? Because there is no information in NVIDIA datasheets and they are annoyingly silent in their own developer forums.