And if you are a non-science major (or possibly even for science majors), throw in a few upper-division science for good measure. Schools like to see that.
From a practical standpoint, however many you need to graduate as a Faculty of Science student and thensome (if you need classes to fill up your schedule). I took the bare minimum of 7 (3 core + 4 elective) and opted to graduate early. We're on the quarter system here.