Do they require a letter of recommendation for undergraduate admissions? It doesn’t really specify…
It looks like they take one from a teacher or counselor. You might check the USC forum here to see what others have done.
http://www.usc.edu/admission/undergraduate/firstyear/prospective/checklist.html
Look at the requirements of specific majors, or even schools. I don’t know about all majors, but I had to submit three.