Save an array of values in the post meta box

Just save the array to post meta as you save string-

$data = array("200", "201", "202");

//Update inserts a new entry if it doesn't exist, updates otherwise
update_post_meta($post_ID, '_parent_id', $data);

The for getting the post call get_post_meta like below-

get_post_meta($post_ID, '_parent_id', true);

You’ll get the array. Then run for or foreach loop to show the array data as you want to show.