i7-6700 should handle all 20 at 15 FPS at under 50% CPU usage if you use direct to disc and hardware acceleration and have enough RAM (8 GB minimum, more if you can get it cheap)
Best to keep CPU usage as low as possible for energy efficiency's sake, but since i7 has hyperthreading, that means CPU usage goes up a lot faster beyond 50%. You do need some breathing room for other Windows processes (updates, windows defender, etc) and for remote viewing and clip decoding / exporting.