Use folk for people in general (folk in the country); use folks for relatives or immediate family (my folks), addressing a general crowd (help those folks, will you?), and the second-person plural (you folks). A casualism in either sense, folk and folks are both mass nouns.
Do not use folx. While it attempts inclusion, it is built off a word that has never been exclusionary, leading 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.