Need help with adding custom menu content

For inject <span> into menu, you do it using below code:

//Setup this code into your current function.php file
add_filter('wp_nav_menu_items','add_span_in_menu', 10, 2);
function add_span_in_menu( $items, $args ) {
    if( $args->theme_location == 'primary')  {

        $myContent="YOURCONTENT"
        $items .=  '<li><span class="number">' . $myContent .  '</span></li>';

    }
    return $items;
}

Above snippet will add <span> with content to your WordPress menu. Hope this help you well!