$ which hwprefs 
/usr/bin/hwprefs

[ preferred: ]
$ hwprefs -v cpu_count
 * 2 processors available *
$ hwprefs cpu_count
2
$ hwprefs cpu_disable=2
1
$ hwprefs cpu_count
1
$ hwprefs cpu_enable=2
2

[ alternatively: ]
$ hwprefs cpu_count=1
1
$ hwprefs cpu_count
1
$ hwprefs cpu_count=3
ERROR: insufficient number of physical processors available (2)
$ hwprefs cpu_count=2
2
