I've read that x264 has core limits...8,12?
BUT I've found that it seems to be some kind of data density that makes more cores be used...
A 4K file @ 30mbps will use almost all my 3950x's 16 cores with a "VerySlow" x264 Preset.
and a light dvd.mpg will use 94% of the 16 cores/32threads if I use a "UltraFast" x264 Preset... 2200fps output, very dense data input/output.
I've never read anything with details of how this works.