wrap a span tag around author’s post count

It doesn’t appear that wp_list_authors() supplies an appropriate hook to modify anything unfortunately. The next best thing would be to just do a string replace and hope for the best :/

$start_wrapper="<span class="author-post-count">";       // Set our wrapper start tag
$end_wrapper="</span>";                                // Set our wrapper end tag
$author_html    = wp_list_authors( array(                   // Get Author HTML
    'optioncount'   => true,
    'echo'          => false,                               // Ensure we return and do not echo ( default is TRUE )
) );
$author_html    = str_replace( '</a> (', "</a> {$start_wrapper}(", $author_html );
$author_html    = str_replace( '</li>', "{$end_wrapper}</li>", $author_html );
echo $author_html;

The above will attempt to replace around the anchor tag HTML so you have a proper wrapper. At which point you can echo or append $author_html where you see fit.