Most of the upper division physics courses at UCSD are 4 units each, according to http://www.ucsd.edu/catalog/courses/PHYS.html . So a normal course load of 15-16 units would mean taking 4 such courses if you took all physics courses. If you take 2-3 physics courses in a quarter, you would have 8-12 units of physics courses, leaving you schedule space for 1-2 other courses for 3-8 units.