Can the number of tickets be modified up or down depending on how the size of the group develops?

Changing a booking to a lower number of tickets is only possible if you have the booking as a reservation. You can change/cancel it to another number of tickets up to 4 days before arrival. Additional tickets can always be booked in the Reseller Portal, if still available. Have you already paid for the booking? Then changes can only be made by cancelling your current booking free of charge via the Reseller Portal no later than 4 days before the arrival date. You can then make a new booking, if still available.