When trying to randomly draw 10 circles, it only draws less

I'm using sfml to draw the images.

If I try to draw 10 circles on random positions on the grid:

for (int i{ 0 }; i < 10; ++i)
{
    grid[getRandomNumber(grid.size())] = -1;
}

//later
else if (grid[currentIndex] == -1)
{
    Food food{ sf::Vector2f(x * PIXEL_WIDTH, y * PIXEL_WIDTH) }; // Draws circle
    window.draw(food.getFood());
}

I get something like this:

I get only less than what is expected. It is the same for whenever I change the number of circles I want in the for loop.

I draw the grid like so:

//Food.cpp
#include "Food.h"

Food::Food(sf::Vector2f pos)
{
    m_food.setPosition(pos);
    m_food.setFillColor(sf::Color::Red);
    m_food.setRadius(800 / 64);
}

sf::CircleShape Food::getFood() { return m_food; }

// Main.cpp
int main()
{
    sf::RenderWindow window{ sf::VideoMode(SCREEN_WIDTH, SCREEN_HEIGHT), "Snake" };

    sf::Event event{};

    std::vector<int> grid(PIXEL_WIDTH * PIXEL_HEIGHT);

    Snake snake{ sf::Vector2f(PIXEL_WIDTH, PIXEL_HEIGHT) };

    for (int i{ 0 }; i < 10; ++i)
    {
        grid[getRandomNumber(grid.size())] = -1;
    }

    while (window.isOpen())
    {
        while (window.pollEvent(event))
        {
            switch (event.type)
            {
            case sf::Event::Closed:
                window.close();
            }
        }

        window.clear(sf::Color::Black);

        sf::VertexArray tile;

        tile.setPrimitiveType(sf::LinesStrip);
        tile.resize(PIXEL_WIDTH * PIXEL_HEIGHT * 4);

        for (int x{ 0 }; x < (SCREEN_WIDTH / PIXEL_WIDTH); ++x)
        {
            for (int y{ 0 }; y < (SCREEN_HEIGHT / PIXEL_HEIGHT); ++y)
            {
                    
                int currentIndex{ x * PIXEL_WIDTH + y };
                if (grid[currentIndex] == 0)
                {
                    sf::Vertex* quad = &tile[(currentIndex) * 4];
                    quad[0].position = sf::Vector2f(x * PIXEL_WIDTH, y * PIXEL_HEIGHT);
                    quad[1].position = sf::Vector2f((x + 1) * PIXEL_WIDTH, y * PIXEL_HEIGHT);
                    quad[2].position = sf::Vector2f((x + 1) * PIXEL_WIDTH, (y + 1) * PIXEL_HEIGHT);
                    quad[3].position = sf::Vector2f(x * PIXEL_WIDTH, (y + 1) * PIXEL_HEIGHT);

                    window.draw(quad, 4, tile.getPrimitiveType());
                }
                else if (grid[currentIndex] == -1)
                {
                    Food food{ sf::Vector2f(x * PIXEL_WIDTH, y * PIXEL_WIDTH) };
                    window.draw(food.getFood());
                }

            }
        }
        window.display();
    }
    return 0;
}

I'm not sure as to why it does not produce 10 circles on the grid. I already set the values of those positions to -1 then I look for any values with -1 and draw them in their respective position.