Automatically block bookings when a max per-day booking limit is hit.

max-bookings-for-rtb.php

<?php
/**
 * Plugin Name: Max Bookings for Restaurant Reservations
 * Plugin URI: http://themeofthecrop.com
 * Description: Modifies the Restaurant Reservations plugin to rejects bookings if a max bookings limit has been reached for the day.
 * Version: 1.0
 * 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
 *
 * Text Domain: max-capacity-for-rtb
 * Domain Path: /languages/
 *
 * 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
 */
if ( ! defined( 'ABSPATH' ) )
	exit;

if ( !class_exists( 'mxbfrtbInit' ) ) {
class mxbfrtbInit {

	/**
	 * The single instance of this class
	 */
	private static $instance;

	/**
	 * Path to the plugin directory
	 */
	static $plugin_dir;

	/**
	 * URL to the plugin
	 */
	static $plugin_url;

	/**
	 * Maximum bookings per day
	 */
	public $max_bookings = 2;

	/**
	 * Create or retrieve the single instance of the class
	 *
	 * @since 0.1
	 */
	public static function instance() {

		if ( !isset( self::$instance ) ) {

			self::$instance = new mxbfrtbInit;

			self::$plugin_dir = untrailingslashit( plugin_dir_path( __FILE__ ) );
			self::$plugin_url = untrailingslashit( plugin_dir_url( __FILE__ ) );

			self::$instance->init();
		}

		return self::$instance;
	}

	/**
	 * Initialize the plugin and register hooks
	 */
	public function init() {

		if ( !defined( 'RTB_BOOKING_POST_TYPE' ) ) {
			return;
		}

		add_action( 'init', array( $this, 'load_textdomain' ) );

		// Validate form fields
		add_action( 'rtb_validate_booking_submission', array( $this, 'validate_booking_submission' ) );

	}

	/**
	 * Load the plugin textdomain for localistion
	 * @since 0.0.1
	 */
	public function load_textdomain() {
		load_plugin_textdomain( 'max-bookings-for-rtb', false, plugin_basename( dirname( __FILE__ ) ) . "/languages/" );
	}

	/**
	 * Validate the booking submission to block bookings if max
	 * bookings reached
	 */
	public function validate_booking_submission( $booking ) {

		$party = absint( $booking->party );

		// Check if we have space for this booking
		$capacity = $this->get_day_capacity( $booking->date );
		if ( $capacity === 0 ) {
			$booking->validation_errors[] = array(
				'field'		=> 'date',
				'error_msg'	=> 'Booked to capacity',
				'message'	=> __( 'Sorry, we are fully booked that day. Please visit us another day.', 'max-bookings-for-rtb' ),
			);
		}
	}

	/**
	 * Get the remaining capacity for a given day
	 */
	public function get_day_capacity( $date ) {

		$date = new DateTime( $date );

		$bookings = new WP_Query(
			array(
				'posts_per_page' => 1000, // large upper limit
				'post_type' 	 => RTB_BOOKING_POST_TYPE,
				'post_status'	 => 'confirmed',
				'date_query'	 => array(
					array(
						'year'	=> $date->format( 'Y' ),
						'month'	=> $date->format( 'm' ),
						'day'	=> $date->format( 'd' )
					)
				)
			)
		);

		if ( !$bookings->have_posts() ) {
			return $this->max_bookings;
		}

		$capacity = $this->max_bookings - $bookings->found_posts;

		return max( $capacity, 0 );
	}
}
} // endif;

/**
 * This function returns one mxbfrtbInit instance everywhere
 * and can be used like a global, without needing to declare the global.
 *
 * Example: $ssfrtb = mxbfrtbInit();
 */
if ( !function_exists( 'mxbfrtbInit' ) ) {
function mxbfrtbInit() {
	return mxbfrtbInit::instance();
}
add_action( 'plugins_loaded', 'mxbfrtbInit' );
} // endif;

Hi @ankitkumarsah, I'm no longer involved with the Restaurant Reservations plugin. You may want to contact the current plugin maintainers at https://wordpress.org/plugins/restaurant-reservations/.