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.