If you have a reservation for a party of 11 can you remove let's say 5 if that party decided to do something different or will I have to cancel the entire reservation and rebook?
From my understanding along as some one in the party shows up for the reservation there won't be a cancellation fee charged to your credit card on file so you don't really have to cancel the trip and rebook it. It may be a good idea to call the Disney dining line and let them know that your party downsized that way if it's a popular restaurant someone else may be able to get a reservation.
If you know ahead of time (enough to avoid the cancellation fee) PLEASE call and have them MODIFY your reservation. I had this done when some of our party was not going to make it to Disney as early as they though and had to change a party of 8 to a party of 3. Just make sure they MODIFY it and do not cancel and try to rebook- you could end up losing your reservation that way!