Note: I have read Controlling many LEDs with few GPIO pins, but I can't buy additional parts (Degree project)
The end result is to make a noughts and crosses grid with 2 LEDs per grid cell. This means I will need to be able to light up 18 LEDs individually. Therefore wiring two LEDs in parallel in each cell means I can light each LED depending on the direction of voltage bias applied.
This means I will need a total of 18 RPi connections, however there are only 17 GPIO pins.
I have looked at Charlie- and Multi-plexing and a couple of other LED matrices, but none of these seem to be suitable.