第一步：通过meta_box在文章编辑页面的右边栏添加指定按钮。

效果：

源码：

function pickup() {
    new set_pickup();
}
 
if ( is_admin() ) {
    add_action( 'load-post.php',     'pickup' );
    add_action( 'load-post-new.php', 'pickup' );
}

class set_pickup { 
    public function __construct() {
        add_action( 'add_meta_boxes', array( $this, 'add_meta_box' ) );
        add_action( 'save_post',      array( $this, 'save'         ) );
    }
    // 添加meta_box
    public function add_meta_box( $post_type ) {
        //假设自定义文章类型是pickup
        $post_types = array( 'pickup');
 
        if ( in_array( $post_type, $post_types ) ) {
            add_meta_box(
				'allowstick',
				__( 'Sticky post', 'nv' ),
				array( $this, 'render_allowstick' ),
				$post_type,
				'side',
				'high'
			);
        }
    }
    // 生成容器
    public function render_allowstick( $post ){
		wp_nonce_field( 'myplugin_inner_custom_box', 'myplugin_inner_custom_box_nonce' );
		$pickup_sticky = get_post_meta( $post->ID, '_pickup_sticky', true );
		if ( $pickup_sticky ) {
			if ( checked( '1', $pickup_sticky, false ) )
				$active = checked( '1', $pickup_sticky, false );
		}
		?>
                    <div class="plus_items_list">
                        <label for="pickup_sticky">
            <input id="pickup_sticky" name="pickup_sticky" type="checkbox" value="1" <?php echo $active; ?> />&nbsp;Select to stick</label>
                    </div>
                    <?php
        
    }
    // 保存及更新
    public function save( $post_id ) { 
        if ( ! isset( $_POST['myplugin_inner_custom_box_nonce'] ) ) {
            return $post_id;
        } 
        $nonce = $_POST['myplugin_inner_custom_box_nonce']; 
        if ( ! wp_verify_nonce( $nonce, 'myplugin_inner_custom_box' ) ) {
            return $post_id;
        }
        if ( defined( 'DOING_AUTOSAVE' ) &#038;&#038; DOING_AUTOSAVE ) {
            return $post_id;
        } 
        if ( 'page' == $_POST['post_type'] ) {
            if ( ! current_user_can( 'edit_page', $post_id ) ) {
                return $post_id;
            }
        } else {
            if ( ! current_user_can( 'edit_post', $post_id ) ) {
                return $post_id;
            }
        } 
        $push_pickup_sticky = $_POST[ 'pickup_sticky' ]; 
        update_post_meta( $post_id, '_pickup_sticky', $push_pickup_sticky );
    }
}

第二步：通过meta_key区分置顶与非置顶的自定义文章。通过减法输出指定数量的文章。

源码：

$maxNum = 5;
$sticky_count = 0;
// 获取pickup这个自定义文章中置顶的文章ID，以数组的形式输出。
function post_id_by_meta_key( $meta_key ) {
	$outside = array();
	$args = array(
		'post_type' => 'pickup',
		'tax_query' => array(
			'taxonomy' => 'cat-pickup',
			'field' => 'slug',
			'terms' => 'pick-up',
		),
		'meta_key' => $meta_key,
		'meta_value' => '1'
	);
	$query = new WP_Query( $args );
	while ( $query->have_posts() ): $query->the_post();
	$outside[] = $query->post->ID;
	endwhile;
	return $outside;
}
// 循环条件
function define_loop_args( $present_post_type, $maxNum, $sticky_toggle = 0 ) {
	$loop_args = array(
		'post_type' => $present_post_type,
		'tax_query' => array(
			'taxonomy' => 'cat-pickup',
			'field' => 'slug',
			'terms' => 'pick-up',
		),
	);
	if ( $sticky_toggle == 1 ) {
		$loop_args[ 'meta_key' ] = '_pickup_sticky';
		$loop_args[ 'meta_value' ] = '1';
		$loop_args[ 'posts_per_page' ] = $maxNum;
	} else {
		$array = post_id_by_meta_key( '_pickup_sticky' );
		$loop_args[ 'post__not_in' ] = $array;
		$loop_args[ 'posts_per_page' ] = ( $maxNum );
	}
	return $loop_args;
}
?>
<ul class="pickup-posts">
	<?php
	// 置顶文章循环
	$arg = define_loop_args( 'pickup', '', 1 );
	$query = new WP_Query( $arg );
	while ( $query->have_posts() ): $query->the_post();
	?>
	<!-- 被循环的内容<li></li> -->
	<?php
	$sticky_count++;

	endwhile;
	wp_reset_postdata();
	// 非置顶文章循环
	$arg = define_loop_args( 'pickup', ( $maxNum - $sticky_count ), 0 );
	$query = new WP_Query( $arg );
	while ( $query->have_posts() ): $query->the_post();
	?>
	<!-- 被循环的内容<li></li> -->
	<?php
	endwhile;
	wp_reset_postdata();
	?>
</ul>