What factors influence the number of threads that should be generated per system?

It is dependent on the system's hardware.

On a 2-3 GHz CPU, for example, 400-600 threads can be produced. It also depends on the elements included in your test strategy.

The higher the number of processors and XML processing components, the higher the CPU burden and, as a result, the fewer threads. Load testing with multiple machines is recommended for large loads.