There are 4 AP Physics classes, although schools generally don’t offer all 4.
AP Physics 1 is almost always a first physics course.
AP Physics C :Mechanics almost always requires AP Physics 1 or Physics CP or H (plus calculus concurrently or as a prereq). If you want to see stressed kids, look at the ones that somehow managed to take AP Physics C as their first physics course.
AP Physics 2 usually has AP Physics 1 as a prereq
AP Physics C: E&M usually has AP Physics C: Mech as a prereq (or both mech and E&M are taught as a single course, but with 2 AP exams)