Maximum boost would be +12 and maximum cut would be -12. When you run Audyssey it is Level matching your speakers and sub (among other things) to your Listening Position and when you raise the "gain or trim" you increase the number, and the opposite when you lower it....+12 is the highest it will go and -12 is the step before it shuts down the signal.
You want to stay away from both extremes and it depends what volume (on the subwoofer) you start it at. If you get a -12 reading after running Audyssey then you would want to lower the volume on the sub and try again.
I run a Klipsch/Denon Home Theater and as an example my levels are set at: Left Front -5/Center -3.5/Right Front -5/subwoofer -7.5, etc.