I think I may be a little confused about what you’re trying to do.
An ACF Image field only allows selecting a single image. And as far as I know a post can only have one post_thumbnail add_post_meta( $post_id, '_thumbnail_id', $image );
To select multiple images you’d need to use a repeater to allow adding multiple images in the form.
If you want to add multiple attachments to a post then you should look at https://codex.wordpress.org/Function_Reference/wp_insert_attachment.
If I’m misunderstanding please explain a bit more about what it is you’re trying to do.
