Identify the number of employees in Northern sales region.
Northern sales region refers to RegionDescription = 'Northern'
SELECT COUNT(T2.EmployeeID) FROM Territories AS T1 INNER JOIN EmployeeTerritories AS T2 ON T1.TerritoryID = T2.TerritoryID INNER JOIN Region AS T3 ON T1.RegionID = T3.RegionID WHERE T3.RegionDescription = 'Northern'