Add the numbers to the table used as the combo's RowSource. Use a query to order descending by the number. Change the combo so that it shows both columns. The problem with this method is that only the number will show when the dropdown is closed. You might want to concatenate the number and the text in the query and that way both will show when you close the combo.
There are posts here that include instructions for generating sequence numbers in queries. If your recordset is very small, they might prove useful. You'll need to search for them since I don't have any links saved.