You can get the number of posts in the current loop from the $wp_query
object. How you do it will depend on whether you use the main loop or a custom one.
Main loop
Use the global query object.
<?php
global $wp_query;
$is_single_post = ($wp_query->post_count === 1);
?>
<?php if ($is_single_post): ?>
<div class="one-post"></div>
<?php else: ?>
<div class="multi-post"></div>
<?php endif; ?>
Custom loop
Use your own query object.
<?php
$args = array(
// fetch post type and whatnot
);
$query = new WP_Query($args);
?>
<?php if ($query->have_posts()): ?>
<?php if ($query->post_count === 1): ?>
<div class="one-post"></div>
<?php else: ?>
<div class="multi-post"></div>
<?php endif; ?>
<?php endif; ?>