Allow the admin to set any party size when adding or editing bookings.

admin-any-party-size-for-rtb.php

<?php
/**
 * Plugin Name: Allow Admin Override of Party Limits for Restaurant Reservations
 * Plugin URI: http://themeofthecrop.com
 * Description: Allow the admin to set any party size when adding or editing bookings.
 * Version: 0.0.1
 * Author: Theme of the Crop
 * Author URI: http://themeofthecrop.com
 * License:     GNU General Public License v2.0 or later
 * License URI: http://www.gnu.org/licenses/gpl-2.0.html
 *
 * This program is free software; you can redistribute it and/or modify it under the terms of the GNU
 * General Public License as published by the Free Software Foundation; either version 2 of the License,
 * or (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without
 * even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
 *
 * You should have received a copy of the GNU General Public License along with this program; if not, write
 * to the Free Software Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA
 */
defined( 'ABSPATH' ) || exit;

add_filter( 'rtb_form_party_options', 'aaoplrtb_admin_party_size' );
function aaoplrtb_admin_party_size( $options ) {

	if ( !is_admin() ) {
		return $options;
	}

	global $rtb_controller;

	$party_size_min = (int) $rtb_controller->settings->get_setting( 'party-size-min' );
	$min = empty( $party_size_min ) ? 1 : $party_size_min;
	$max = apply_filters( 'rtb_party_size_upper_limit', 100 );
	$options = array();
	for ( $i = $min; $i <= $max; $i++ ) {
		$options[$i] = $i;
	}

	return $options;
}

add_action( 'rtb_validate_booking_submission', 'aaoplrtb_allow_any_party_size' );
function aaoplrtb_allow_any_party_size( $booking ) {
	if ( current_user_can( 'manage_bookings' ) ) {
		foreach( $booking->validation_errors as $key => $error ) {
			if ( $error['field'] == 'party' ) {
				unset( $booking->validation_errors[ $key ] );
			}
		}
	}
}