The seven segment LED board can now drive 16 LED using only an Arduino Nano. Using the circuit described below it can drive 18 LEDs using a stock Arduino Uno/Nano and 20 if the LED on pin D13 is removed by desoldering it.
Here is a picture of the board showing the first 16 digits of PI
The first board gets two Arduinos. The first one, at the top of the board sends instructions to the second one at the bottom of the board. The bottom arduino is responsible for receiving serial instructions and driving the LEDs, If a packet received through the serial port is not meant for the current board, it gets relayed to the next board. The TX pin in the bottom Arduino is connected to the RX pin on the next board's Arduino Nano.