Hello!

How to calculate a Motor or an engine RPM while it running?

Not sure what your asking, but the standard way of measuring is to use an inductive pickup on one of the plug wires.

It used to be that number of cylinders and the whether the engine was 2 or 4 cycle was enough. In some cases you need to know if the plug fires twice (exhaust and after compression). You can also interrogate the ECM.