Yeah, you will be fine with an i7-4790 you have laying around. Just make sure you run substreams and do every optimization in the
wiki.
I have a 4th gen SFF I got off the refurb list of a business lease and I am running over 30 cameras ranging from 2MP to 8MP, running OpenALPR for two cameras, the plate utility to log the plates, and DeepStack on 5 cameras, with most of the cameras running at 15FPS, but a few are down in the 10-12 range and one is at 20FPS. Recording 24/7 substream with mainstream on triggers and not running hardware acceleration.
I have optimized the system fully to the
Wiki. At night it will sip in the single digits. In the daytime it is in the teens. It will jump with OpenALPR in the daytime.
I rebooted my machine last week and I leave this program running to monitor temps, CPU%, memory, etc. It shows what it is at now and the max it has seen since the last reboot. I wish it showed the min also, but oh well. I am remote desktop in and that adds a few percent, but you can see it is sitting in the single digits at night and had a max of 30ish% for the past week.
So yeah, I think 20 cams on your system will be fine LOL...