33 Concurrency support library [thread]

33.4 Threads [thread.threads]

33.4.3 Class thread [thread.thread.class]

33.4.3.7 Static members [thread.thread.static]

unsigned hardware_concurrency() noexcept;
Returns: The number of hardware thread contexts.
[Note 1: 
This value should only be considered to be a hint.
— end note]
If this value is not computable or well-defined, an implementation should return 0.