byte pinCount = 20;

void setup() {
  for (byte pin = 0; pin < 20; pin++) {
    pinMode(pin, OUTPUT);
  }
}

void loop() {
  // Turn on LEDs from first to last
  for (byte pin = 0; pin < pinCount; pin++) {
    digitalWrite(pin, HIGH);
    delay(100); // Adjust delay as needed
    digitalWrite(pin, LOW);
  }

  // Turn on LEDs from last to first
  for (int pin = pinCount - 1; pin >= 0; pin--) {
    digitalWrite(pin, HIGH);
    delay(100); // Adjust delay as needed
    digitalWrite(pin, LOW);
  }
}