$i = 1;
foreach ($seriesgroup as $item){
    if($i == 1 or $i == 2 or $i == 20){ // i 1 or 2 or 20
        echo '<div>';
        foreach ($item as $sub_item){
            echo '<span>'.$sub_item.'</span> <br>';
        }
        echo '</div>';
    }
    $i++;
}