We booked at 16 months...as soon as the
point charts were released for the cruise dates that we wanted... We could've booked prior to the charts being released - actually, as soon as the cruise itineraries were announced, but I wanted to know what the points were going to be first. In our case,
DCL announced their itineraries several weeks before
DVC released the point charts. Since the point charts fluctuate wildly from one year to the next we decided to wait for the charts to be sure we could do it.
So basically, you can book with points as soon as DCL announces the itineraries, but you may not know for sure how many points you will really end up needing. They will let you cancel w/o penalty I believe if you end up not being able to meet the point requirements once the charts are released.
OP - you will have to talk to MS about the additional points for the 5th / 6th persons. Standard cabins only hold 4 people - so you will either have to book a suite or 2 connecting cabins in a lower category. When you pay with cash the 2 cabins are the cheaper option. My guess would be that you will need to book 1 adult in one cabin and then 2 or 3 others...then book another adult in the other cabin and the remaining guests. So you will follow the point requirements based on how many people you book in each cabin. DCL will not care about your final sleeping arrangements...but you need to BOOK 1 adult in each room.
