Among the customers from Chicago, Illinois, what is the highest quantity of products bought in a single order?
from Chicago refers to City = 'Chicago'; Illinois refers to State = 'Illinois'; highest quantity refers to max(Quantity)
SELECT T1.Quantity FROM west_superstore AS T1 INNER JOIN east_superstore AS T2 ON T1.`Customer ID` = T2.`Customer ID` INNER JOIN central_superstore AS T3 ON T3.`Customer ID` = T2.`Customer ID` INNER JOIN south_superstore AS T4 ON T4.`Customer ID` = T3.`Customer ID` INNER JOIN people AS T5 ON T5.`Customer ID` = T4.`Customer ID` WHERE T5.City = 'Chicago' AND T5.State = 'Illinois' ORDER BY T1.Quantity DESC LIMIT 1