Translation:How many people are there on the beach?
As I understand it, folk is when the emphasis is on the group, not the individuals within it. Personer shifts the focus onto the individuals who make up the group.
So in this example, asking how many personer are on the beach is a request for precise numbers (lifeguards, for example, might need that kind of information). Whereas asking how many folk there are would be more vague and impressionistic, and would get an answer such as 'mange' or 'ikke mange'.
In my (far from perfect) understanding of English, I think that it would have been better with "how many persons are there on the beach", and I woud say "there are people on the beach". So, in MY usage of English, "people" is mostly uncountable.
In Norwegian, "folk" is uncountable. "Det er folk på stranden. " When ypu start counting them, they are "personer".