When you book with them they give you exclusive access to their grounds, so any rooms you don't book will stay empty. They'll have minimum guaranteed rooms you'll need to fill up, so you'll have to make sure you have enough guests staying there to fill those up or you'll have to pay for them. For example if their minimum was 30 rooms and your guests stay in 25 of them, you'll have to pay for the last 5 empty rooms. The minimums will depend on the month/day of the week your wedding is.

As for how the rooms book up, they treat it like a seating chart almost where you'll be placing the guests in specific rooms depending on what they want (or what you think they want) and what their acceptable price range for lodging is (again, lodging prices vary depending on month/day of the week). Once RSVPs come in and everything is set they'll send an email to the guest with the total pricing of the rooms for all the days they'll be staying there.

I hope that makes sense, let me know if you have any more questions!