Eight. A full feature solution would include FE trim, FE grade, center defect trim, cut-in-two, NE grade, NE trim, fence, and scale-off. All these features can be used at the same time on a single board, and some will require more than one symbol - in fact up to six symbols could be used for any feature.
So although it isn't likely, as many as 48 symbols could be used on any one board at a time. Most boards are marked with fewer than three characters, resulting in an average solution time of less than 6 milliseconds.