You would have your room number already because you booked into an assigned cabin. Those that booked into a "Guarantee" won't know until closer to the cruise. There are pros and cons of booking either way. Those who book in guarantee may get upgraded, but it can be the very front of the ship, very back of the ship, less desirable room by noise, etc. they don't know where they will be or what deck. By selecting an assigned room you can choose if you want midship, what deck, etc. Some people are particular about those things so it is in their advantage to do it that way. Others who aren't as particular take the chances with the "guarantee" method.