Wow this is like kryptonite for danish people. I am really struggling with the answer to this question, as it is something danish people just know.

"Mennesker" basically means humans, while "folk" basically means people. However in English when you would say "there are many people here", you wouldn't in danish say "der er mange folk her". In that context you would use "mennesker". There are many examples like this, so it is really confusing. There's a good explanation on the following site but it is in Danish so maybe that won't help you sadly. http://basby.dk/aftenhold/dagpapir/mennesker%20eller%20folk.pdf

Thanks Oliver! It helps!