How many "Mountain-100 Silver, 38" were sold by Stearns MacFeather?
Mountain-100 Silver, 38' is name of product;
SELECT SUM(T2.Quantity) FROM Employees AS T1 INNER JOIN Sales AS T2 ON T1.EmployeeID = T2.SalesPersonID INNER JOIN Products AS T3 ON T2.ProductID = T3.ProductID WHERE T1.FirstName = 'Stearns' AND T1.LastName = 'MacFeather' AND T3.Name = 'Mountain-100 Silver, 38'