30 Thread support library [thread]

30.3 Threads [thread.threads]

30.3.2 Class thread [thread.thread.class]

30.3.2.6 Static members [thread.thread.static]

unsigned hardware_concurrency() noexcept;
Returns: The number of hardware thread contexts.
[Note
:
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.