@Rita you can set in your.spiceinit file (the one in the directory where ngspice runs) this:

set num_threads=4

However ngspice is not a true parallel simulator. The matrix solver can not be parallelized. However since each simulation step consists of 2 diffferent main tasks: equation solver and device equations evaluation/update it will effectively use 2 threads (or more) to cut model equations evaluation time. However from the manual: _"If OpenMP is enabled, but num_threads not set, a default value num_threads=2 is set internally"_ so i guess you won't see significant advantages.