Of the customers who are from Canada, how many used Federal Shipping?
from Canada refers to Country = 'Canada'; Federal Shipping refers to ShipName = 'Federal Shipping'
SELECT COUNT(T3.CustomerID) FROM Shippers AS T1 INNER JOIN Orders AS T2 ON T1.ShipperID = T2.ShipVia INNER JOIN Customers AS T3 ON T2.CustomerID = T3.CustomerID WHERE T2.ShipName = 'Federal Shipping'