Customising the WC REST API v2 Order Number

functions.php

<?php 

// the code below goes in your functions.php file

function my_custom_prepare_shop_order_object($response, $order, $request) {
  $data = $response->get_data();
  if( function_exists('wc_seq_order_number_pro') ) {
    $data['number'] = wc_seq_order_number_pro()->find_order_by_order_number( $data['id'] );
  }
  $response->set_data($data);
  return $response;
}

add_filter( 'woocommerce_rest_prepare_shop_order_object', 'my_custom_prepare_shop_order_object', 20, 3 );