How many employees have territories in the Eastern region?
"Eastern" is the RegionDescription
SELECT COUNT(DISTINCT T1.FirstName) FROM Employees AS T1 INNER JOIN EmployeeTerritories AS T2 ON T1.EmployeeID = T2.EmployeeID INNER JOIN Territories AS T3 ON T2.TerritoryID = T3.TerritoryID INNER JOIN Region AS T4 ON T3.RegionID = T4.RegionID WHERE T4.RegionDescription = 'Eastern'