OMP_NUM_THREADS– Specifies the number of threads to use
Specifies the default number of threads to use in parallel regions. The value of this variable shall be a comma-separated list of positive integers; the value specifies the number of threads to use for the corresponding nested level. Specifying more than one item in the list will automatically enable nesting by default. If undefined one thread per CPU is used.
OpenMP specification v4.5, Section 4.2