Benchmarks, at the time, showed that performance started to fall off due to contention if the number of processes got much larger. I imagine that the speed of storage today would maybe make 3 or 4x core count a pretty reasonable place to start. There will be a point of diminishing returns somewhere, but you can probably construct your own benchmarks to determine where that point is likely to be for your workload.
I wonder if anyone has run benchmark like that lately? Doing such a benchmark maybe worth while given that so much is now running either in the cloud or running in a VM or some other kind of Container. all this abstraction from the hardware layer surely has had to have an impact on the numbers and rules of thumb...