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Private Sub SeuBotão_Click()
'By JPaulo ® Maximo Access
Dim rst As DAO.Recordset
Dim i As Integer

Set rst = CurrentDb.OpenRecordset("SELECT * FROM SuaTabela")

rst.MoveFirst

i = 1
Do
   If IsNull(rst!SeuCampoN) Or rst!SeuCampoN = "" Then
    rst.Edit
    rst!SeuCampoN = i
    rst.Update
    rst.MoveNext
    i = i + 1
    Else
         rst.MoveNext
   End If
    Loop Until rst.EOF
    rst.Close
Set rst = Nothing
End Sub

Private Sub SeuBotão_Click()
'By JPaulo  Maximo Access
Dim rst As DAO.Recordset
Dim i As Integer
Dim strUP As String

Set rst = CurrentDb.OpenRecordset("SELECT * FROM SuaTabela")
'busca o ultimo numero gerado, se não existir inicia no 1
strUP = Nz(DMax("[SeuCampoN]", "SuaTabela"), 1)
                    
rst.MoveFirst

i = strUP
Do
  If IsNull(rst!SeuCampoN) Or rst!SeuCampoN = "" Then
   rst.Edit
   rst!SeuCampoN = i
   rst.Update
   rst.MoveNext
   i = i + 1
   Else
        rst.MoveNext
  End If
   Loop Until rst.EOF
   rst.Close
Set rst = Nothing
End Sub