A colleague asked me this question, after a read of the man page for psrinfo, came up with the following commands. The -p gives a count and -pv gives more information.
su78sr28# psrinfo -p
su78sr28# psrinfo -pv
The physical processor has 64 virtual processors (0-63)
UltraSPARC-T2 (chipid 0, clock 1415 MHz)