Hi Von,
TotalDataSourceItemCount will return -1 until the last page is fetched via a call to MoveToLastPage.
TotalItemCount will return -1 until all pages are fetched or the TotalDataSourceItemCount is known.
While TotalDataSourceItemCount returns the count of records available to be returned from the datasource, TotalItemCount will also include items only present in the client.
If you want to retrieve the total item count before reaching the last page, you will need to query this value:
var totalCount = mgr.Employees.Count();
Regards,
Silvio.