LED strip as seven segment display, 3 LEDs per segment.

strippy.ino

#include <Adafruit_NeoPixel.h>

#define PIN 8

Adafruit_NeoPixel strip = Adafruit_NeoPixel(24, PIN, NEO_GRB + NEO_KHZ800);

// LED strip, 3 LEDs per segment, standard 7 segment LED layout

unsigned long DigitBytes[] = {
 0x0003FFFF, // 0
 0x000001F8, // 1
 0x001C7E3F, // 2
 0x001C0FFF, // 3
 0x001F81F8, // 4
 0x001F8FC7, // 5
 0x001FFFC7, // 6
 0x000001FF, // 7
 0x001FFFFF, // 8
 0x001F81FF  // 9
};

void setup() {
  strip.begin();
  strip.show();
}

void showDigit(int d) {
  unsigned long digit = DigitBytes[d];
  
  for (int i = 0; i < 21; i++) {
    if (digit & 1) {
      strip.setPixelColor(i, 0xFF0000);
    } else {
      strip.setPixelColor(i, 0x000000);
    }
    digit = digit >> 1;
  }
  strip.show();
}

void loop() {
  for (int x = 0; x < 10; x++) {
    showDigit(x);
    delay(5000);
  }
}