I just noticed that while generating ambient occlusion maps (masking) all the CPU’s on my dual quad core (Mac Pro) are working but not while creating a displacement map. Not sure if this is a setting that needs to be turned on/changed or if that is how it works right now.
There are no settings in 3.12 for adjusting the use of multiple processors. ZBrush will detect the number of processors/cores that you have and use what it can (or needs to) at any given time.
For technical reasons, some features may not use all processors at this time.