Just a heads up, the number of cores isn’t the only consideration, it’s the speed and modern architecture. There’s old xeon multi cpu servers floating around with many many cores but run dog slow as plotters. just something to keep in mind. The fastest plots come in around 4-5hours each and are all from AMD Ryzen systems.
3400 is the “medium” suggestion from this February blog post, which also says the maximum usable -b (for k=32, 128 buckets, bitfield on) is 6750. With how much RAM OP has, I see no reason to go below that. (No idea if versions after the post changed anything on this front, though)
In my testing adding memory “improved” plot time from 6:22 to 6:18 … so I wouldn’t even bother adding memory at all unless you have literally no other use for it. It’s like 1-2 percentage point faster plotting at best.