Echo the number of posts being displayed

rearrange your code and use post_count;
example:

    <?php 
        $movieNumber = 1;
        $today = date('Ymd');
        $posts = array (
            'post_type' => 'movie',
            'posts_per_page'   => -1,
        );
        $loop = new WP_Query( $posts ); ?>
    <div id="posters<?php echo $loop->post_count; ?>">
    <?php while ( $loop->have_posts() ) : $loop->the_post();
...etc...