Changing dead cells to alive with rand

void initArea(Area* a, unsigned int n)
{
    unsigned int living_cells, max_living_cells, y, x;
    living_cells = 0;
    max_living_cells = n;
    srand(time(NULL));
    while (living_cells <= max_living_cells)
    {
        x = (rand() % (a->xsize));
        y = (rand() % (a->ysize));
        a->cells[y][x] = ALIVE;
        living_cells++;
    }
}

I'm trying to make some of my dead cells alive with rand(), but when I have to make for example 50 alive cells, this code always gives little bit less. Why?