This snippet counts the total number of products purchased in the order and outputs the total below the order details table.
add_action( 'woocommerce_email_after_order_table', 'ecommercehints_show_number_of_products_purchased_in_emails', 10, 4 );
function ecommercehints_show_number_of_products_purchased_in_emails( $order, $sent_to_admin, $plain_text, $email ) {
echo 'Number of products purchased: ' . $order->get_item_count() . '
';
}