Modify text after post count

The _n() function will sort out single from plural. How to use it is in the codex.

EDIT:
Apologies. My first go at an answer wasn’t quite enough to get you there. The trouble with using wp_list_categories() in this context is that it prints out pre-formatted HTML and you don’t end up with any value you can use for $count.

Instead, try using get_categories() which will return an array of category objects you can tap into to get post counts and display accordingly.

Something like the following should do the trick, or at least get you started:

<?php
$categories = get_categories();
echo '<ul>';
    foreach($categories as $category) { 
        echo '<li><span class="cat-count"><a href="' . get_category_link( $category->term_id ) . '">' . $category->name.'</a>'; 
        echo ' ' . $category->count . _n(' Entry', ' Entries', $category->count ) . '</span></li>';
    } 
echo '</ul>';
?>