We've gone in different groupings - even bringing a non-family member along once (older kids' friend). ; It can get confusing with more people, however it is nice to have enough adults to be able to split up. ; There is always someone that doesn't want to go on a ride that someone else does, so there can be alot of waiting around if you can't split. ; Just make sure you have a way to communicate between groups - it's hard to estimate the time it'll take you to do a certain ride and you want to make sure you meet up again.
We've had wonderful vacations just ourselves and also as an extended family group of 10!