Sub addCustomSlideNumber()

    On Error Resume Next
    Dim PPSlide As Slide
    For Each PPSlide In ActivePresentation.Slides
        PPSlide.Shapes("customNumberBox").Delete
    Next

Dim x As Integer
Dim slideToNumber As Slide
Dim oShape As Shape

For Each slideToNumber In ActivePresentation.Slides
  If slideToNumber.SlideShowTransition.Hidden = msoFalse Then
    x = x + 1
    slideToNumber.HeadersFooters.SlideNumber.Visible = msoFalse
    Set oShape = slideToNumber.Shapes.AddTextBox(msoTextOrientationHorizontal, 890, 500, 50, 14)

    With oShape
      .TextFrame.TextRange.Text = CStr(x)
      .TextEffect.FontName = "Palatino"
      .TextEffect.FontSize = 10
      .TextEffect.Alignment = msoTextEffectAlignmentRight
      .Name = "customNumberBox"
End With

  Else
    slideToNumber.HeadersFooters.SlideNumber.Visible = msoFalse
  End If
Next

End Sub

Sub deleteTextBox()
    On Error Resume Next
    Dim PPSlide As Slide
    For Each PPSlide In ActivePresentation.Slides
        PPSlide.Shapes("customNumberBox").Delete
    Next
End Sub