In the Reseller Portal, you can modify or cancel the number of reserved tickets up to 4 days before your arrival.
Have you already paid for the booking? In that case, changes can only be made by cancelling the current booking for free via the Reseller Portal, up to 4 days before your arrival date. After that, you can create a new booking with the correct number of tickets.
You can always add extra tickets in the Reseller Portal, provided tickets are still available for the selected time slot.