How to display serial numbers in foreach loop while querying posts?

How about:

echo '<ul>';

$sn_count = 1;
foreach($urls as $url){
    echo '<li><span class="serial-number">'.$sn_count.'</span> '.$url .'</li>';
    $sn_count++;
}

echo '</ul>';

You can then style your serial numbers with the serial-number class.