20x(20-1) = 380 leds (charlieplexing, high low); Correct me if i'm wrong. That means you are able to control 380 "Whatever" using diodes and one Arduino.

First, the figure of 380 is correct for 20 pins (20² - 20) however pins 0 and 1 are connected to pull-up resistors on the Atmega16U2, so they are never really at 0V. If you load a blank sketch, you will measure 5 V on pins 0 and 1.

Second, you can't drive 380 "whatevers" you can drive 380 LEDs via Charlieplexing. The property of the LED, that current only flows one way through it, is important to the way it works.

Related question Is there a way to have more than 14 Output pins on arduino?

20x(20-1) = 380 leds (charlieplexing, high low); Correct me if i'm wrong. That means you are able to control 380 "Whatever" using diodes and one Arduino.

First, the figure of 380 is correct for 20 pins (20² - 20) however pins 0 and 1 are connected to pull-up resistors on the Atmega16U2, so they are never really at 0V. If you load a blank sketch, you will measure 5 V on pins 0 and 1.

Second, you can't drive 380 "whatevers" you can drive 380 LEDs via Charlieplexing. The property of the LED, that current only flows one way through it, is important to the way it works.

Related question Is there a way to have more than 14 Output pins on arduino?