First you crack open the back of the box...be sure you have your radiation suit on, thar's plutonium in there, I swear!!
Just kidding...WRKSYSACT will tell you active processors in the LPAR, if you're using multiple LPAR's, if not, it tells you total number of active processors.
Your question though, might be how many total, iSeries systems sometimes have more physical processors than are active for the processor-on-demand system. WRKHDWRSC *PRC will show you the physical processors, active or not.