How to set a limit on the number of cores used for compilation/running

When one runs a NLO process, locally or on a cluster, the code in the various P** dir is compiled on the local machine. For complicated processes with many P** dirs, this can lead to the usage of all the cores for a non negligible amount of time, with the corresponding slow down/blocking of the other users/processes.
The number of core to be used for compilation or local running can be set via the configuration variable nb_core, by setting this variable to the maximum number of cores that can be used.
As for all settings, this can be done either globally, in the input/mg5_configuration.txt file, or in a process specific manner, by editing the PROC_DIR/Cards/amcatnlo_configuration.txt file.
We recall that in these configuration files, the line corresponding to the variable to be set must be uncommented.