How many NVIDIA NVENC engines a particular GPU have?
Since you are actually more interested in performance (Frame Per Second) than in the NVENC engines count, please consider my below rounded up figures.
I've compared GTX 1060 and GTX 1080 only.


H264, GTX 1080 encodes 70%  faster than GTX 1060
HEVC, GTX 1080 encodes 140% faster than GTX 1060