Display any voucher numbers from voucher items in the order on the New Order admin email. To use this snippet simply paste into the bottom of your theme's functions.php

wc-pdf-product-vouchers-voucher-number-on-admin-email.php

<?php

// Displays any voucher numbers on the New Order admin email
add_action( 'woocommerce_email_order_meta', function( $order, $is_admin ) {

	if ( $is_admin ) {

		$voucher_numbers = array();
		$order_items = $order->get_items();

		if ( count( $order_items ) > 0 ) {
			foreach ( $order_items as $item ) {

				if ( $item['product_id'] > 0 && isset( $item['voucher_id'] ) && $item['voucher_id'] ) {

					$voucher = new WC_Voucher( $item['voucher_id'], $order->id, $item );

					$voucher_numbers[] = $voucher->get_voucher_number();

				}
			}
		}

		echo "<p><strong>Email Certificate Number:</strong> " . implode( ', ', $voucher_numbers ) . "</p>";

	}

}, 10, 2 );