You need the argument post_per_page
to limit the posts.
Before your loop it should be something like this.
$args = array (
'post_type' => 'custom_post',
'posts_per_page' => '10',
);
$wp_query = new WP_Query( $args );
// your loop
if ( $wp_query ->have_posts() ) {
while (have_posts()) : the_post();
get_template_part( 'preview', get_post_format() );
endwhile;
}
wp_reset_postdata();
If it is the custom post, make sure to replace custom_post