How to query users by post count no less than 10

I can see in your code you did mistakes with meta key arguments. You must use underscore instead of hyphen sign.
Here is corrected code:

$args = array(
            'number' => 5 ,
            'meta_key' => 'post_count',
            'meta_value' => 10,
            'meta_compare' => '>',
            'orderby' => 'rand'
            );

            $user_query = new WP_User_Query( $args );

            if ( ! empty( $user_query->get_results() ) ) {
                foreach ( $user_query->get_results() as $user ) {

                    echo  $user->display_name ;

                }
            } else {
                echo 'No users found.';
            }

Hope this will work!