Are you referring to the serial number stamped into the side of the wooden stocks? Many, if not most of the stocks only have a portion of the serial number on them, usually the last 4-5 digits of the SN. There are some that only have 3 digits and some that have 6 or more. I don't recall which year or arsenal it was that put the whole SN on the stock, but it did happen. Seems like it was an 8 digit SN on the side of that stock.
As far as the "highest" SN recorded on a stock, it would have to be one of the ones that used the entire SN. Like a 21-24 million number. RM could probably give you a more detailed, accurate answer on this.