My current system is running with 16GB of ram. The one it replaced had 32GB, but it seemed to be under-utilized, so when I upgraded, I went with less memory. Had no noticeable affect.
As with anything, unless you unlimited horsepower (CPU), you have to budget your processing power and data bandwidth (pipe between CPU and storage). If your data stream from 10 cameras is more than your bus can handle writing to the drive, you'll start having issues even if the CPU can handle the image processing.
While 4K resolution is pretty to look at on a big TV/monitor, the reality is, a single 4K camera will really tax the CPU. I don't recall the math, but when dealing with system limitations, it's a function of total megapixels and frame rate.
For example, a single 4K camera runs at 8MP per image. Getting the clean crisp images you see when directly logged into the camera is very difficult to achieve from an NVR - because the data stream tends to be reduced and highly compressed. Thus, digitally zooming in on a 4K image provides a lot of potential, once it's been compressed to meet reasonable storage and CPU requirements, most of the benefits of 4K evaporate.
If you have only one or two 4K cameras, you probably can try to maintain higher bitrates and your CPU would not be too stressed. But anything more than a handful, and even the higher-end systems will cry uncle.
Consider also that, generally, as resolution increases, sensitivity to light decreases. That means as sunlight fades, and certainly at night, the performance will drop off. High res cameras will compensate with higher gain, or slower shutter speeds (or a combination of both) and that results in noisy images, and blurred motion.
The temptation to have fewer high-res cameras than more low-res is understandable. But the reality is, your PC will really struggle to process the higher MPs and your night vision will be dramatically worse.
Most of my cameras are capable of 3 or 4MP. I keep them at 2MP for common/higher-traffic areas, and 1MP (720P) in areas that have low value/interest. With the lower MP, I can add cameras as needed without adding significant load to the Blue Iris machine.
The fact that your high-res cameras can be de-tuned to lower resolutions is a good thing, though the sensors will still under-perform in low-light conditions. But at least you'll have some wiggle room when your BI machine starts to scream for mercy
For what it's worth, there is a ton of very good information at ipcamtalk.com. Lots of people with years of experience - along with camera vendors and manufacturers - that can provide guidance.
I originally started with VHS tape systems. Then 8-ch analog boards on old Pentium PCs. Then IP cameras. And now HD IP PoE cameras. And while you can buy boxed proprietary systems from consumer-level suppliers (like Lorex, Swann, etc), you're going the best way. Dedicated machine, open cameras, and a well supported NVR program that will work with nearly any brand and type of camera out there.
Good luck!