How many of these people cancelled their lesson?
SELECT count ( DISTINCT T2.staff_id )  FROM Lessons AS T1 JOIN Staff AS T2 ON T1.staff_id   =   T2.staff_id WHERE T1.lesson_status_code  =  "Cancelled"
count of distinct people refers to COUNT(DISTINCT column_name); people refers to staff_id; cancelled their lesson refers to lesson_status_code = 'Cancelled';