This is highly likely in Peak Season (December - April), you'd be lucky if there was a bus that wasn't full...

You have a two options, which the reservations team will go through with you when you try to book your bus. 

- Option One -

Get your name put on the standby list and wait to see what happens. Usually this is your best bet and generally it will sort its self out.

- Option Two -

Reservations can advise you when the next available bus is and book you onto it if you want a confirmed seat.