As I said, with a macro. For example, the following macro will output a word-count for each Section at the end of the document.
Code:
Sub Demo()
Dim Sctn As Section, StrCounts As String
With ActiveDocument
For Each Sctn In .Sections
StrCounts = StrCounts & "Section " & Sctn.Index & ":" & vbTab & _
Sctn.Range.ComputeStatistics(wdStatisticWords) & vbCr
Next
.Range.InsertAfter StrCounts
End With
End Sub