How many products that take more than 2 days to make are out of stock?
take more than 2 days to make refers to DaysToManufacture>2; out of stock refers to OnOrderQty = 0 or OnOrderQty is null
SELECT COUNT(T2.ProductID) FROM ProductVendor AS T1 INNER JOIN Product AS T2 ON T1.ProductID = T2.ProductID WHERE T1.OnOrderQty IS NULL OR T1.OnOrderQty = 0