How many classes are professor whose last name is Graztevski has?
SELECT count(*) FROM employee AS T1 JOIN CLASS AS T2 ON T1.EMP_NUM  =  T2.PROF_NUM WHERE T1.EMP_LNAME  =  'Graztevski'
number of classes = COUNT(*); professor with last name Graztevski refers to EMP_LNAME = 'Graztevski'; classes are related to professor through EMP_NUM = PROF_NUM relationship;