Sub AutoOpen()
Dim oVars As Variables
Dim bVar As Boolean
Dim lngCount As Long
    Set oVars = ActiveDocument.Variables
    For Each oVar In ActiveDocument.Variables
        If oVar.Name = "varNum" Then
            bVar = True
            lngCount = oVar.Value + 1
            Exit For
        End If
    Next oVar
    If Not bVar Then lngCount = 1
    oVars("varNum").Value = lngCount
    UpdateAllFields
    ActiveDocument.Save
lbl_Exit:
    Exit Sub
End Sub

Private Sub UpdateAllFields()
Dim oStory As Range
    For Each oStory In ActiveDocument.StoryRanges
        oStory.Fields.Update
        If oStory.StoryType <> wdMainTextStory Then
            While Not (oStory.NextStoryRange Is Nothing)
                Set oStory = oStory.NextStoryRange
                oStory.Fields.Update
            Wend
        End If
    Next oStory
    Set oStory = Nothing
lbl_Exit:
    Exit Sub
End Sub