There is a MAX_STRINGS constant in ./src/org/herac/tuxguitar/gui/actions/track/TrackPropertiesAction.java, maybe changing that will already do the trick.
You also may need do something in "setDefaultTuning()"
to fill the strings with any default value.
or add some "Default" to the switch like:
default:
for( int i = 1 ; i <= this.stringCount ; i ++ ){
this.tempStrings.add(TGSongManager.newString(getSongManager().getFactory(),i,0));
}
break;
it should be enougth for TuxGuitar,
But you may get some problems on file format exporters.
on GP* file formats, there is no support for more than 7 strings. so you'll not able to save the track to any GP format.
on TG format there is no limitation to save them ( well it's a byte, so you can't have more than 127 )