I could try running some stuff on my personal PC to try to get a baseline. It has an i7 7700, 64GB ram, and a 3070. I should at least be able to see if this is enough to handle most of it. There are certain aspects I can't really test just yet.
You'll be CPU limited, but yes, that would be a great place to start. That 3070 should be fine for all but future-AV1 encoding, so will be a fine test in terms of NVENC (using H.264 at the moment), etc.
One thing to be aware of is USB Root Hub overload (in this case, too many cameras/ too much bandwidth, especially when looking at multiple 4K60). if you are looking at a workstation class machine, less of an issue, but even then you may have to be careful.
Beware mixing camera connection technologies (USB, HDMI, NDI, etc) and ending up with varying video feed camera latency. so 1. make sure camera latency is same (not necessarily easy to do), then computer has to decode all that incoming traffic (I'm not exactly sure if/how to control using GPU decode vs CPU). Getting super technical, and depending on exactly how incoming video appears to OS and drivers, etc, then maybe?? will impact if you need a GPU with multiple NVENC decoders?... I'm don't know... I could be way off base, but something for you to research further and confirm one way or the other
As for thinking of using USB cameras, especially that high of resolution models, beware cable length limits.
As for overall PC specs, what is common is that you will figure out what you want/need, and there will be a level of CPU/GPU resource requirements to accomplish that. Then, the natural tendency is to tweak/improve that, increase sophistication, etc.. and that stuff can (usually does) increase h/w resource demands. How much, no way to know.
So you risk either overbuy/overspend now, or having to buy yet another more powerful system way before the end-of-life with the 1st system ... or get it just right... total crap shoot. Personally, I'm the type to overspend on somethign really good, then take care of it for a long time, and get great value. but that is not always the wise choice, especially with where PC technology is at the moment (mid inflection). If your environment is something where spending 50% less now, and then replacing in 2+ years if perfectly acceptable (you have confidence there won't be 'budget constraints' pushing livestream operator with trying to make due with something that is overloaded) that could easily be the smarter move. Some places operate more of 'this is a special project, purse strings open this once' ... so it depends
The 14th gen Intel HX CPUs are slightly warmed over 13th gen chips. And nice discounts are available on 13th gen workstation systems now that 14th gen CPUs released. Personally, I'd like to get a mobile workstation with PCIe v5 SSD and Thunderbolt 5 ports, but that won't be until the end of this year, at earliest (from Tier 1 vendors). so rather than continuing to wait, it probably makes sense for me to step outside my comfort zone and get a discounted 13th gen system now and plan to replace it way sooner than I have any other personal system (ever)