It is very common that in configurations that uses numbers of cores there is the auto magic word that means all available cores on the machine. auto in this case should be meant as $( nproc )
It's not documented as far as I know but, if MAKEOPTS is entirely unset, portage currently defaults to -j$(nproc) if 'MAKEOPTS' not in mysettings: nproc = get_cpu_count() if nproc: mysettings['MAKEOPTS'] = '-j%d' % (nproc)