I am crunching on 4 cores using my OnePlus 5T: it remains cold, doesn't drain the battery too fast and perfectly usable for other things. Before that I have tried with 8 cores and full speed: it gets too warm and apparently Android OS (8/9) keeps killing the processes because of internal purposes.

Therefore I recommend to run 4 tasks simultaneously. If you realy want fast crunching, then a desktop system based on, for instance, AMD Ryzen 2700x will give you 16 threads, which will overcome any Android phone like a Ferrari against a Fiat Multipla.