You can think of it like an optimization problem. With a given computer system how can you shape the workload to make best use of the system resources available? You would need to know which system resources and how much of each are used in the plotting process. The best way to answer this question would be with direct measurements on your own hardware but for a starting point, you can look at the defaults set by the GUI:

2 threads, 4608 MiB memory, and 332 GiB temporary space.

After you have the baseline resource usage for 1 plotting process you can begin to estimate the requirements for multiple simultaneous plotting processes.

For a naive answer, you would multiply each of these requirements by the number of parallel plotting processes that you want to run simultaneously. The real answer is more complicated because not all of the resources are needed throughout the entire plotting process. For instance, 332 GiB of temporary space is only required for a relatively short percentage of the time. You can take advantage of this by staggering your plotting processes so that they are not all in the max-temp-space part of the process at the same time. You can find periods of high and low usage for CPU and RAM resources as well. You job would be to then find the best compromise for your system in terms of ensuring system resources are being used to their fullest but not overloading the system such that everything grinds to a halt.