You can set the number of processors by typing a new number of processors into the Default number of processors text field. When the number of processors is greater than 1, VisIt will attempt to run the parallel version of the compute engine. You can also click on the up and down arrows next to the text field to increase or decrease the number of processors. If VisIt finds a parallel host profile, you will have the option of changing the number of processors before the compute engine is actually launched.