Post X of Y in single.php / sidebar.php

class MY_Post_Numbers {

    private $count = 0;
    private $posts = array();

    public function display_count() {
        $this->init(); // prevent unnecessary queries
        $id = get_the_ID();
        echo sprintf( '<div class="post-counter">Post number<span class="num">%s</span><span class="slash">/</span><span class="total">%s</span></div>', $this->posts[$id], $this->count );
    }

    private function init() {
        if ( $this->count )
            return;
        global $wpdb;       
        $posts = $wpdb->get_col( "SELECT ID FROM $wpdb->posts WHERE post_status="publish" AND post_type="post" ORDER BY post_date " ); // can add or change order if you want 
        $this->count = count($posts);

        foreach ( $posts as $key => $value ) {
            $this->posts[$value] = $key + 1;
        }
        unset($posts);
    }

}
$GLOBALS['my_post_numbers'] = new MY_Post_Numbers;

function my_post_number() {
    $GLOBALS['my_post_numbers']->display_count();
}

to use in template file :

<?php my_post_number(); ?>