https://codex.wordpress.org/Class_Reference/WP_Query#Properties

示例代码：

<?php
$args = array( \'post_type\' => \'your_custom\' );
$custom_query = new WP_Query( $args );

if( $custom_query->have_posts() ) {
   $number_of_posts = $custom_query->found_posts; 

   while( $custom_query->have_posts() ) { 
      $custom_query->the_post();

      if( $number_of_posts == 1 ) { 
         the_content(); 
      } else { 
         the_excerpt(); 
      }

   } 

   wp_reset_postdata();

}
?>

请尝试此代码（&P）；让我知道结果（我还没有测试）

<?php
$post_query = new WP_Query(\'post_type=post\'); // replace the post type with your post type key
$total_posts_found = $post_query->found_posts;

if($total_posts_found < 2 && $total_posts_found > 0){
    if($post_query->have_posts()) : while($post_query->have_posts()) : $post_query->the_post();
        echo \'<h1>\'get_the_title().\'</h1>\';
        echo get_the_content();
    endwhile;
    endif;
    wp_reset_postdata();
} else {
    if($post_query->have_posts()) : while($post_query->have_posts()) : $post_query->the_post();
        echo \'<h1>\'get_the_title().\'</h1>\';
        echo get_the_excerpt();
    endwhile;
    endif;
    wp_reset_postdata();
}
?>