ระบุจำนวนลูกค้าที่ไม่มีประวัติการชำระเงิน
List the number of customers that did not have any payment history.
df_customers = pd.DataFrame(columns=['customer_id'])
df_customer_payments = pd.DataFrame(columns=['customer_id'])
df_customers[~df_customers['customer_id'].isin(df_customer_payments['customer_id'])].shape[0]