I think this should do it:

Sub Number_Items()
    Dim lngIndex As Long: lngIndex = 1
    Dim lngIndex1 As Long: lngIndex = 1
    Dim oLastRow As Long, i As Long
    Dim lCol As Long
    For lCol = 4 To 8
        oLastRow = Cells(Rows.Count, 4).End(xlUp).Row
        For i = 9 To oLastRow
            With Cells(i, lCol)
                .Value = Replace(.Value, "DXX", lngIndex)    ' << Search and Replace here
                lngIndex = lngIndex + 1
            End With
        Next i
    Next
End Sub