A casualism in either sense, folk and folks are both plurals. Reserve folk for general people (folk in the country) and folks for relatives or immediate family (my folks), addressing a general crowd (help those folks, will you?), and the second-person plural (you folks).
Although I prefer to avoid playing linguistic politics, do not use folx; while it attempts inclusion, it’s built off a word that has never been exclusionary, leading many readers to view it as a sign of ignorant moral posturing rather than any act of empathetic inclusion. The content of a writer’s ideas does far more to convey the writer’s nuanced beliefs than do cheap spelling tricks, which anyone can apply without real empathy toward other people.