If Query Post Returns 1 Post

You should not use query_post when you can use the mutch faster query WP_Query.

You can count the post with the property post_count.

Here is a snippet that counts your post in the loop:

<?php

$args = array(
    'post_type'     =>  'post',
    'cat'           =>  '20',
    'post_status'   =>  'publish',
    'order'         =>  'DESC'

    );

// The Query
$the_query = new WP_Query( $args );

// The Loop
while ( $the_query->have_posts() ) : $the_query->the_post();

    if( ( $the_query->post_count <= 1 ) {
        echo 'One or less';

    } else {

        echo 'More than one post';
    }

endwhile;

// Reset Post Data
wp_reset_postdata();

?>