By default, AMS engines will try to use all available cores on your machine. One exception is that ReaxFF jobs with less than 1000 atoms set up in the GUI will run in serial by setting export NSCM=1 in the run script.
You can limit the number of cores yourself, by setting the NSCM environment variable.
In the GUI, you can change the default number of cores the Sequential queue uses from AMSJobs -> Queue -> Edit -> Sequential and change Default Options to the number of desired CPU cores for most of your jobs.
You could also change the Interactive queue, which will enable you to run multiple jobs simultaneously at a subset of the available CPU cores.
You can still overwrite this default behavior in AMSjobs by entering a different number of cores in the rectangle to the right of the job name and queue, which then sets the # of cores for that job if you run it.