You can't increase it
It prints everything it has stored and once it loses something it is gone forever.
With most machines (I can't find specifics for the 6180) it does the following.
It will print a report of the last 99 jobs, or the faxes that happened since the last report was printed.
Now if you have sent more than the 14, are are 100% sure that no report was printed it could be:
Firmware was updated
Memory was cleared/reset
Power failure and it lost the info