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...