Multi-threaded (aka parallel) applications take advantage of several cores...
but, currently, almost no desktop application is multi-threaded. Mozilla,
with its dozens of developers, is only starting to split Firefox into *two*
processes: https://wiki.mozilla.org/Electrolysis
Going sequential to multi-thread means rewriting code. And the new code is
much harder to write. And not all tasks benefit from parallelizing in the
first place.
Notice however that several cores allow to perform several heavy tasks (such
as transcoding) and still have a reactive system: launching one more
application will be fast as long as at least one core still is mostly idle.
But that is not common on a desktop system. That is why I would say four
cores already is many (again: for a *desktop* system).
