Probably a dumb question, but is there a list of the maximum number of columns and rows that each Teensy variant could support?
Number of available pins I/O pins = the sum of number of rows + number of columns. The Teensy 2.0 has 25 pins and the Teensy++ 2.0 has 46.
The Phantom uses all pins on the Teensy 2.0, with a 17×6 matrix and 2 LEDs. (17+6+2 = 25)
Note that the built-in LED is also on pin D6. On the Phantom, this LED is pulsing very fast all the time, faster than the eye can see.
It could instead have been a 8×13 matrix, and left the D6 pin alone, but the routing of that would have been a little bit more complex.