The final table is determined to be the table with the most seats at it. This is due to the fact that you can always move people around, seat players who aren't even in the tournament, add/delete tables, add/remove seats, etc, at any time, so the software could determine which table was the actual final table, but it could very easily be error-prone.
So, to set the number of players who made the 'final table', just make sure the largest table configured has X number of seats. No need to move players, and this can be done after the fact. Load up any saved tournament, delete all tables, create a new table with X seats, save the tournament.