I would think it would have only one formal night i am going on the 13 night cruise on the destiny in just over a week and think there will only be one formal night as there is no other passengers getting on the ship after maderia. Hope you enjoy the transatlantic crossing we thought of doing it but could not stand the 6nights crossing so we are doing the cruise in january for 2 weeks.
I am also going on destiny for 13 nights and have been told by travel agent that there will be 2 formal nights really looking forward to it.
The clue here is 'weekly' ....thomson cruises have one formal night per week of the cruise. On our 13 night cruise in october we had two formal nights!!