My apologize the wrong code has been edited, you edited to have the order number for a status update.
To have the order number in the order creation mail, in the same file replace:
$order->mail->subject = trim($order->mail->subject);
if(empty($order->mail->subject)) {
$order->mail->subject = JText::sprintf('NEW_ORDER_SUBJECT',$order->order_number,HIKASHOP_LIVE);
}
By:
$order->mail->subject = '';
if(empty($order->mail->subject)) {
$order->mail->subject = JText::sprintf('NEW_ORDER_SUBJECT',$order->order_number,HIKASHOP_LIVE);
}