The Touch Board has 12 touch electrodes positioned along the top side of the board that have been placed for easy access. You can connect to these electrodes either with crocodile clips and wires or by directly painting to the pads with Electric Paint or some other conductive paint or ink (see updates for our "Connecting to the Touch Board" post).
As it stands you can draw out 12 lines (or connect 12 wires) and create 12 sensors to use as touch events and trigger associated MP3 files. This is what we did to create the Alphabet Demo seen in our video and hence why the letters stopped at L.
However, one can create many more touch keys by using the simultaneous triggering of 2 electrodes as a unique touch event, thus making up to 66 different combinations. This would require a matrix of sorts and would need a insulating layer between touch pads placed on top of each other, and also between any traces that cross other traces leading up to the sensors.
Another way to obtain higher resolution is by making a touch pad in which case you would use a standard touch pad row and column configuration. This, however, would require some processing capability to accurately map the capacitance values, which might need to happen on the computer with the Touch Board working as a serial communication device for the touch data.