As it says, go to the file that is shown below, which is qa-include/db/maxima.php. You should do that before setting up Q2A. If you already set it up you'll have to update the database manually.
By the way, the limit will be increased in Q2A 1.8 to 12000. It can be increased some way beyond that, but not too much or you'll hit the MySQL maximum row limit. Probably around 18000 is the absolute maximum, but Q2A does not use that in case we want to add more fields in the future.