Example of how to add a column that lists the current number of regs vs the total number of spaces currently available on the event

tw_ee_add_pending_payment_registrations_count_column.php

<?php //Please do not include the opening PHP tag if you already have one.

//This code adds a column for 'Pending Payment Registrations'.
function tw_ee_add_pending_payment_registrations_count_column( $columns, $screen ) {
    $columns = EEH_Array::insert_into_array(
        $columns,
        array( 'registrations_pending_payment' => '<span class="dashicons dashicons-groups ee-icon-color-ee-blue ee-icon-size-20"><span class="screen-reader-text">' . esc_html__('Pending Payment Registrations', 'event_espresso') . '</span></span>' ), 
        'actions'
    );
    return $columns;
}
add_filter( 'FHEE_manage_toplevel_page_espresso_events_columns', 'tw_ee_add_pending_payment_registrations_count_column', 10, 2 );

function tw_ee_add_pending_payment_registrations_count_column_data( $event, $screen ) {
    if ( ! $event instanceof EE_Event ) {
        return '';
    }
   	$pending_payment_regs = $event->count_related(
      'Registration',
      array(
        array(
          'REG_deleted' => 0,
          'STS_ID'      => EEM_Registration::status_id_pending_payment,
        ),
      )
    );
    echo $pending_payment_regs;
}
add_action( 'AHEE__EE_Admin_List_Table__column_registrations_pending_payment__toplevel_page_espresso_events', 'tw_ee_add_pending_payment_registrations_count_column_data', 10, 2 );

tw_ee_add_registrations_sold_total_column.php

<?php //Please do not include the opening PHP tag if you already have one.

//This code adds an additional column 'Regs sold/total'.
function tw_ee_add_registrations_sold_total_column( $columns, $screen ) {
    $columns = EEH_Array::insert_into_array(
        $columns,
        array( 'registrations_sold_total' => 'Regs sold/total' ), 
        'actions'
    );
    return $columns;
}
add_filter( 'FHEE_manage_toplevel_page_espresso_events_columns', 'tw_ee_add_registrations_sold_total_column', 10, 2 );

function tw_ee_add_registrations_sold_total_column_data( $event, $screen ) {
	if ( ! $event instanceof EE_Event ) {
        return '';
    }
   	$registered_attendees = EEM_Registration::instance()->get_event_registration_count($event->ID());
    $total_spaces = $event->total_available_spaces() == EE_INF ? '&infin;' : $event->total_available_spaces();
   	echo $registered_attendees . '/' . $total_spaces;
}
add_action( 'AHEE__EE_Admin_List_Table__column_registrations_sold_total__toplevel_page_espresso_events', 'tw_ee_add_registrations_sold_total_column_data', 10, 2 );

// Add some CSS to style the column 
add_action( 'admin_enqueue_scripts', 'tw_ee_custom_events_columns', 20 );
function tw_ee_custom_events_columns() {
    $css = 
        '#Extend_Events_Admin_List_Table-table-frm .column-registrations_sold_total {
    		width: 10%;
		}';
    wp_add_inline_style('events-admin-css', $css);
}