What are the last names of employees who serve at most 20 customers?
SELECT T1.LastName FROM CUSTOMER AS T1 JOIN EMPLOYEE AS T2 ON T1.SupportRepId  =  T2.EmployeeId GROUP BY T1.SupportRepId HAVING COUNT(*)  <=  20
SELECT T1.LastName FROM CUSTOMER AS T1 JOIN EMPLOYEE AS T2 ON T1.SupportRepId  =  T2.EmployeeId GROUP BY T1.SupportRepId HAVING COUNT(*)  <=  20
[{'LastName': 'Hansen'}, {'LastName': 'Köhler'}]