I'm looking to build my own NVR/Blue Iris server and am wondering if anyone can give me some advice on specs. I plan to have about 20 Hikvision cameras (8MP/4K) recording continuously about medium quality, 20fps, H.265. Does anyone have any suggestions as to what CPU, motherboard, GPU (is this needed?), RAM (amount) this system will require? I have my hard drive storage figured out for the most part based on a calculator.

You have a lot of flexibility but I'd say an Intel i-Series CPU... for 20 cameras I'd recommend an i7 7or i9, the newer the better but make sure it supports Intel QSV. To throw a spec out there, I'd say 8th gen or newer. Motherboard.... anything that supports the CPU you pick. RAM... 12GB or more would be more than sufficient. If using the CPU noted there's no real need to have any add-on GPU. Beyond that... Windows 10/Server 2019 should do it for ya. At the MP/s you're talking I wouldn't recommend a VM because of the difficulty with Intel QSV passthrough to the VM.