one possibility: using the ‘format=array’ and ‘echo=0’ parameters; and building a foreach loop to output each tag:
<ol>
<?php
$wptc = wp_tag_cloud('smallest=12&largest=12&orderby=count&order=DESC&format=array&unit=px&number=5&echo=0');
foreach( $wptc as $wpt ) echo "<li>" . $wpt . "</li>\n";
?>
</ol>