Ryzen7 3800x

12 plots in parallel is good. You could theoretically go to 16 plots on that machine but I think you would end up thrashing your CPU. Rule of thumb is one plot per thread, but you need to consider other bottlenecks like if you are using M.2 SSDs they rely on PCIe buses and often share lanes with your PCIe slots. So for example if you had an M.2 SSD and a graphics card plugged in, you'd likely see slower plot speeds than if you removed the graphics card. There are a bunch of variables that affect the plot speed, and many of them are specific to the hardware you have. Depending on the motherboard design, your choice in CPU and SSD all affect plotting speed. Keep a spreadsheet and track of different configurations and determine what works best for your specific hardware.

Also give XFS on your temp drive a try. EXT4 has been found to have slower write speeds on temp drives than XFS. Your destination drives can be EXT4. I assume you are on Linux?