https://eventespresso.com/topic/problem-sending-tickets-via-email-for-multiple-attendee-registrations/#post-185173

custom_email_output_function.php

<?php
if ( !function_exists( 'event_espresso_custom_questions_output' ) ){
    function event_espresso_custom_questions_output( $atts ) {
    global $wpdb;

     extract( $atts );
	//Get the questions for the attendee

		$sql = "SELECT ea.answer, eq.question FROM " . EVENTS_ANSWER_TABLE . " ea ";
		$sql .= " LEFT JOIN " . EVENTS_QUESTION_TABLE . " eq ON eq.id = ea.question_id ";
		$sql .= " WHERE ea.attendee_id = '" . $attendee_id . "' ";
		$all_questions == TRUE ? '':$sql .= " AND system_name IS NULL ";
		// !empty($show_admin) && $show_admin == TRUE ? '':$sql .= " AND eq.admin_only != 'Y' ";
		$sql .= " ORDER BY eq.sequence asc ";

        $questions = $wpdb->get_results( $sql );
        //echo $wpdb->last_query . '<br />';

        $email_questions = '';
        $q_counter = 0;
        $q_num_rows = $wpdb->num_rows;
        if ( $q_num_rows > 0 )
        {

            foreach ( $questions as $question ) {
                $email_questions .= $question->answer != '' ? wpautop( '<strong>' . $question->question . ':</strong><br /> ' . str_replace( ',', '<br />', $question->answer ) ) : '';
                $q_counter++;
                if ( $q_counter == $q_num_rows )
                   return $email_questions;
            }
        }
        return $email_questions;
    }

}