Does frequency equal the number of steps a stepper motor will do?
The PWM frequency is entirely independent of the step rate*. So, for instance, if you apply a 25 KHz PWM waveform to your stepper motor, and keep the PWM waveform constant, the stepper motor will not turn at all. The "PWM" part is just a way to set the coil current to some intermediate value between full current and zero, and do it efficiently, that is, without wasting heat in a linear driver.


For a PWM rate which is higher than the maximum step rate when not using microsteps.


If you want the smoothest possible rotation, then yes, you want to use every microstep value when changing the current level in the stepper coils, and your calculation is correct. But there is no obvious reason why you need to do this. If you're using microsteps and want to rotate the shaft faster, you can just skip over intermediate microstep values. You'll lose smoothness doing this, but you'll get faster rotation.