A [NEXT_UPCOMING_DATETIME_REG_VALUE] shortcode which counts all of the registrations for all tickets assigned to the next upcoming datetime within the event. By default the shortcode outputs the count of all 'Approved' registrations, you will need to pass the status you prefer to the shortcode such as RPP (Registration Pending Payment).

ee_next_upcoming_datetime_reg_value.php

<?php
/*
Plugin Name: EE Site Specific Plugin
Description: Add custom functions for Event Espresso 4 here.
*/
/* Begin Adding Functions Below This Line; Do not include an opening PHP tag as this sample code already includes one! */

//You can pass the status to the shortcode using [NEXT_UPCOMING_DATETIME_REG_VALUE status="{status}"]
//For example [NEXT_UPCOMING_DATETIME_REG_VALUE status="RPP"] for Pending Payment registrations.

function display_next_upcoming_datetime_reg_value( $attributes = array() )
{	
	//If this is being pulled by the Calendar, get out.
	if ( defined( 'DOING_AJAX' ) && DOING_AJAX )
	{
	    return;
	}
	
	//Pull in all EE reg statuses
	$reg_status_array = EEM_Registration::reg_status_array();

	//Check the give status matches one of known statuses or is equal to 'all'.
	if( (isset($attributes['status']) && $attributes['status'] != 'all') && !isset($reg_status_array[$attributes['status']])) {
		//If the status does not match any known EE status, return invalid.
		return __( 'Invalid status provided', 'event_espresso');	
	}

	//set default attributes
	$default_shortcode_attributes = array(
		'status'		=> EEM_Registration::status_id_approved,
	);

	$attributes = array_merge( $default_shortcode_attributes, (array) $attributes );

	//load helpers
	EE_Registry::instance()->load_helper( 'Event_View' );

	if ( empty( $attributes['event_id']) ) {
		if ( is_single() && is_espresso_event() ) {
			$event = EEH_Event_View::get_event();
		}
	} else {
		$event = EEM_Event::instance()->get_one_by_ID( $attributes['event_id'] );
	}

	if( $event instanceof EE_Event) {

		$datetimes = EEM_Datetime::instance()->get_datetimes_for_event_ordered_by_start_time( $event->ID(), false, false, 1 );
		$datetime = end($datetimes);
		
		$tickets = $datetime->tickets();

		$registration_count = 0;

		foreach ( $tickets as $ticket) {

			if( $attributes['status'] == 'all' ) {
				$registration_count += $ticket->count_registrations( array( array( 'STS_ID' => array( '!=', EEM_Registration::status_id_incomplete ) ) ) );
			} else {
				$registration_count += $ticket->count_registrations( array( array( 'STS_ID' => array( '=', $attributes['status'] ) ) ) );
			}
		}
	}
	return $registration_count;
}
add_shortcode('NEXT_UPCOMING_DATETIME_REG_VALUE','display_next_upcoming_datetime_reg_value');