JetBooking. Connect Guests number filter to Guests field in the form.

jet-booking-inherit-guests-filter-in-form.php

<?php
/**
 * Instructions:
 *
 * 1. Create a filter for guests number - https://tppr.me/H7Sjzs, https://tppr.me/D5jImO
 * 2. In the example below used 'max_guests' query variable for filter, you need replace it with your own. 
 *    You need to do this under 'Replace this with your filter variable' comment
 * 3. After this - each time when this filter applied - selected value will be stored, now you need to apply it in form
 * 4. Set this value as default for Guessts field in the form - https://tppr.me/vwdRCV.
 * 5. For the form keep the query var name from example - _booking_guests_num - here will be stored selected filter value
 */
add_filter( 'jet-smart-filters/query/final-query', function( $query ) {
	
	// Replace this with your filter variable
	$query_var = 'max_guests';
	$store_type = jet_abaf()->settings->get( 'filters_store_type' );

	foreach ( $query['meta_query'] as $index => $meta_query ) {
		
		if ( isset( $meta_query['key'] ) && $query_var === $meta_query['key'] ) {
			jet_abaf()->stores->get_store( $store_type )->set( '_booking_guests_num', $meta_query['value'] );
		}
		
	}
	
	return $query;

} );

add_action( 'init', function() {
	$store_type = jet_abaf()->settings->get( 'filters_store_type' );
	$guests_num = jet_abaf()->stores->get_store( $store_type )->get( '_booking_guests_num' );
	
	if ( $guests_num ) {
		$_GET['_booking_guests_num'] = $guests_num;
	}
} );