There isn't one. Most of the Sun servers don't have a machine readable serial # (aka the system serial # that's printed on the box).
Many admins fix this problem by updating the eeprom oem-banner parameter with this value prior to putting the system in service. So you'll have to go to the remote site and read the value with a flashlight.
how to detect CPU serial number on Solaris platform? is it possible to use a command to get ? or from an API?
Are you asking about getting the chassis serial number for a computer system, or the serial of an individual cpu module?
The earlier replies (I think) are describing a scenario for chassis serials.
I don't happen to be aware that Sun/Oracle (for SPARC and UltraSPARC modules), nor Intel, nor AMD (for their families of x86 architecture products) ever code module serials in a fashion that could be retrievable by a simple OS command.