How to display custom logo on WordPress login page

Change the code to:

function my_login_logo() {
    $site_id = get_current_blog_id();
?>
    <style type="text/css">
        body.login div#login h1 a {
            background-image: url(<?php echo get_stylesheet_directory_uri(); ?>/images/Logo-B-Classic-<?php echo $site_id; ?>.jpg);
        }
    </style>
<?php }
add_action( 'login_enqueue_scripts', 'my_login_logo' );

Then, update the logo image file names to append the site ID accordingly to each different logo image file, for example Logo-B-Classic-21.jpg where ’21’ is the site ID.