How many orders in the south superstore with the fastest delivery are items of furniture?
fastest delivery refers to Ship Mode = 'First Class'; furniture refers to Category = 'Furniture'
SELECT COUNT(T1.`Order ID`) FROM south_superstore AS T1 INNER JOIN product AS T2 ON T1.`Product ID` = T2.`Product ID` WHERE T1.`Ship Mode` = 'First Class' AND T2.Category = 'Furniture'