What temperature rating do I actually need for winter camping?

For genuine winter use, look for a bag rated 0°F or colder; a 20°F bag is generally a shoulder-season or mild-winter choice. Keep in mind that advertised temperature numbers are often 'survival' or comfort-optimistic, so many experienced campers buy a bag rated colder than the lowest temperature they expect and add a quality insulated pad, which matters as much as the bag itself.