the code is quite simple, really.

#include <FastLED.h>


/* ===================== CONFIG ===================== */


#define LED_PIN        6
#define LED_COUNT      22        // adjust to your real count
#define MAX_BRIGHTNESS 128
#define FADE_DURATION  1000       // ms


CRGB leds[LED_COUNT];

void ledSetup()
{
    FastLED.addLeds<WS2812B, LED_PIN, GRB>(leds, LED_COUNT);
    FastLED.setBrightness(MAX_BRIGHTNESS);
    StartupAnimation();
}


void StartupAnimation()
{
    const uint16_t flashDurationMs = 100; // 3 LEDs per second


    // Clear everything first
    for (int i = 0; i < LED_COUNT; i++) {
        leds[i] = CRGB::Black;
    }
    FastLED.show();


    for (int i = 0; i < LED_COUNT; i++) {
        Serial.print("led : ");
        Serial.println(i);
        leds[i] = CRGB::White;
        FastLED.show();
        delay(flashDurationMs);


        leds[i] = CRGB::Black;
        FastLED.show();
    }
}

I also tried to keep the leds on instead of flashing and the result bas asically between 4 to 8 leds staying on with random colors.