We have a reservation for 6 people for this Sunday, actually. I booked my 180 day on April 1 and Sunday was only day 2 of our trip, so I did not get it at first. However, thanks to reservation finders I was alerted a couple months later and got it. I recommend signing up for them.
How far into your trip is Sunday? You'll have better chances getting a reservation the later into your trip it is. On our last trip (only 4 guests) Sunday was 6 days in so it was easier to get.
As long as someone shows up you will not be charged for a missing person.