using correct number of DIMMS for best memory performance Intel Xeon scalable cpu
Solution 1:
Basically, the CPU has the fastest memory access with one DIMM per channel.
Adding more DIMMs increases the load on the memory bus and may decrease the RAM clock.
Dual-rank and quad-rank DIMMs cause higher load and potentially decrease the clock further than fewer ranks. Same size modules with fewer ranks are generally better.
Registered DIMMs (RDIMMs) decrease the bus load, may increase the maximum number of modules and ranks but come with a (slight) penalty in latency.
Load-reduced DIMMs decrease the bus load even further (possibly enabling more DIMMs/ranks or higher clock rate) and have the same latency penalty as RDIMMs.
Unbuffered DIMMs (UDIMMs) are the fastest but only allow very few modules and ranks. Usually they're only found in basic entry-level servers.
Low-voltage DIMMs save energy but the lower voltage swing often decreases the maximum RAM clock that's possible.
Generally, you can't mix UDIMMs, RDIMMs and LR-DIMMs in a system. You can usually mix LV and normal DIMMs but the LV DIMMs will run at the higher voltage.
The exact metrics for your system should be in the manual. There's no one rule for all.
Solution 2:
I'm not sure what you mean by minimum. This would depend on your specific application requirements and the software vendor would be the one to tell you.
The memory slot population will be described in the manual for the server/motherboard. It should have specific instructions on which slots to populate and in which order.
Not fully utilizing the available memory channels will reduce performance but it always depends on what you're running on the server and how dependent it is on memory bandwidth specifically. This might also answer 4. As you increase the number of DIMMs per socket you'll see increased performance until you're at the number of memory channels per CPU. After that performance might degrade slightly as multiple DIMMs have to share the same memory channels.
https://www.pcworld.com/article/2982965/components/quad-channel-ram-vs-dual-channel-ram-the-shocking-truth-about-their-performance.html
I'm basing this on a few assumptions so let me know if you have any questions/corrections.