how to save and get selected item id list with add_meta_box

Question

There are few problems with your code.

First,

The name of the select field is not the same as you are trying to save using the global variable $_POST. You are saving $_POST['selectId'] data but it does not exist in your meta box. You named the select field as “count” but trying to get value using "selectId" key.

Second,

You need to get the saved value and use it to compare with the select field value to be selected.

Your wpdocs_my_display_callback function will be like this to set the selected value.

    function wpdocs_my_display_callback()
    {
        global $post;
        $saved_value = get_post_meta($post->ID, 'selectId', true);
    ?>
    
        <select name="selectId" id="selectId">
            <?php
    
            $posts = new WP_Query(array('posts_per_page' => -1, 'post_type' => 'header'));
            while ($posts->have_posts()) : $posts->the_post(); ?>
                <option <?php selected($saved_value, get_the_ID()); ?> id="selection" value="<?php echo get_the_ID(); ?>"><?php echo get_the_title(); ?></option>
            <?php endwhile;
            // die()
            ?>
        </select>
    <?php
    }

I think it will work for now.