Yes, that does sound right, as it averages to slightly more than 1 unrestricted class per quarter. Among engineers, at least, it’s common to take two engineering class and a third class that is slightly less demanding so as not to overwork one’s self. In fact, the CS department strongly advises against taking more than 2 CS classes in any given quarter:
http://csmajor.stanford.edu/Courseload.shtml
Also, the School of Engineering has four-year plans online for each engineering major. They’re by no means binding, but give you a good idea of what class schedules will look like:
http://web.stanford.edu/group/ughb/cgi-bin/handbook/index.php/4-Year_Plans
Note that the buffer of the additional units also gives you time during your freshman year to shop around. Most of the majors require only two years to complete, with some engineering ones requiring three years. Therefore, each student typically has about one year’s worth of courses that are unrestricted.