you won’t benefit from multicore usage if you don’t use many separated audio flows.
I mean: if you for example use only one VST, as vV said, all data will be processed in a single CPU.
this is how Renoise works: it let’s each CPU process a different audio flow.
more information here
there may be even scenarios where multicore CPU’s perform worse than single core ones, primarily because multithreading has a computation cost by itself