This is documentation for Mathematica 8, which was
based on an earlier version of the Wolfram Language.
View current documentation (Version 11.1)

$ProcessorCount

$ProcessorCount
gives the number of processor cores available on the computer system on which Mathematica is being run.
By default, as many parallel kernels are launched as there are processor cores:
You can change $ProcessorCount in a new session to simulate a multicore computer:
No kernels are launched by default on a single-core computer:
Specify the number of desired kernels to launch them anyway:
Changing $ProcessorCount takes effect only if it happens before using any parallel commands:
You can always launch the desired number of kernels explicitly:
New in 7