How many professors teaches no more than two high-level or harder undergraduate courses?
professors refers to taughtBy.p_id; high-level or harder undergraduate courses  refers to courseLevel = 'Level_400' ; no more than two refers to count(taughtBy.course_id) < = 2
SELECT COUNT(*) FROM ( SELECT COUNT(T2.p_id) FROM course AS T1 INNER JOIN taughtBy AS T2 ON T1.course_id = T2.course_id WHERE T1.courseLevel = 'Level_400' GROUP BY T2.p_id HAVING COUNT(DISTINCT T1.course_id) <= 2 )