Tickets purchased through www.keukenhof.nl can be changed in numbers up to 4 days (from the reserved time slot) before arrival. This can only be done by cancelling the entire booking (costs €2.50 per booking). You can do this by sending an e-mail to info@ticketcounter.nl. You can then book new tickets with the correct numbers, provided they are still available.
Extra tickets can also be ordered in a new booking.