Can mennesker and folk be interchanged?
Det er mye folk i parken. = Det er mye mennesker i parken? What is the difference between mennesker and folk?
In your example they can be interchanged, but "folk" would be the most common choice. You would also have to say "mange mennesker" instead of "mye", because menneske is a countable noun :)
Generally, the difference would be that "menneske" is merely an individual of the human species. "Folk" is more general, and also occurs in some idiomatic expressions like "oppfør deg som folk!" (mind your manners!)