มีผู้บริจาคเงินบริจาคให้กับโรงเรียนชื่อ "เกลนน์" กี่คน?
How many donors have endowment for school named "Glenn"?
school = pd.DataFrame(columns=['school_id', 'school_name']); endowment = pd.DataFrame(columns=['donator_name', 'school_id'])
endowment[endowment['school_id'].isin(school[school['school_name'] == 'Glenn']['school_id'])]['donator_name'].nunique()