Updating post meta after post publish

I am trying to resolve custom post field auto increment field problem.
Using ACF plugin.
I have tried approach to setup custom field named: sifra
I need this field to be auto increment.
I am trying to get max value of post meta filed “sifra” from database and then increment it by 1.
Using “publish_post” hook to update custom field with auto incremented value.

So far this is my code but it doesn’t work:

function update_sifra($post_id) {
    if( ( $_POST['post_status'] == 'publish' ) && ( $_POST['original_post_status'] != 'publish' ) ) {
        $post = get_post($post_id);
        $maxsifra = $wpdb->get_results($wpdb->prepare(
            "
            SELECT MAX(sifra)
            FROM wp_postmeta
            "
        ));
		$nextsifra = $maxsifra++;
		update_post_meta($post, 'sifra', $nextsifra);

    }
}
add_action( 'publish_post', 'update_sifra' );

Please guide me to the right steps to make this work.
Thank you.