how to create dynamic columns in Pivot table

in below example script if i have 10-15 emps i have no idea how many employees exactly, 
so now i need each column as one column where i have no idea how many employees are there
how do i add additional colomn based on no.of employees
please assume 10-15 employees
USE AdventureWorks2008R2;
GO
SELECT VendorID, [250] AS Emp1, [251] AS Emp2, [256] AS Emp3, [257] AS Emp4, [260] AS Emp5
FROM
(SELECT PurchaseOrderID, EmployeeID, VendorID
FROM Purchasing.PurchaseOrderHeader) p
PIVOT
(
COUNT (PurchaseOrderID)
FOR EmployeeID IN
( [250], [251], [256], [257], [260] )
) AS pvt
ORDER BY pvt.VendorID;

HarSan