In PSS form, there is one option called Number of harmonics. I am NOT able to understand what is its use as PSS is any way a Steady state DC operating point analysis ?
Of course, No of harmonics actually controls the time step for better accuracy (like to avoid Aliasing effect etc). But the time step can easily controlled by maxacfreq OPTION.
So my doubt is:- Whether the Harmonic Option in PSS form is needed for Some other reasons ?
IS there any documents which actually explains the need/use (OTHER than controlling Time Step) of No. of Harmonics option in PSS form
It depends on whether you're using shooting or harmonic balance. From your description above, it sounds like shooting.
The main reason is if you actually want to observe the harmonics produced by PSS. That is useful in itself in some cases. If you're only doing the PSS as a means to get a steady state (it's not a "DC operating point") in order to do a small signal analysis (e.g. pnoise, pac, pxf etc), then you don't have to save any harmonics. maxacfreq would be sufficient (assuming you even need to set that; maxacfreq defaults to 4x the highest harmonic requested, or 40x the PSS fundamental, whichever is the higher). So you can generally consider the maxharms to be the number of output harmonics.
For Harmonic Balance however it affects the accuracy - it's quite different.
The SpectreRF Theory document will probably describe this in more detail (I didn't check the documentation to make sure or point you in the right place due to lack of time right now).