Hi, not sure if you got an answer already, but I understand that there are a maximum of 2 formal nights on Carnival cruises. I did a 7 night cruise on the Carnival Triumph and it had 2 formal nights. I have checked for my next cruise (17 nights) on the Carnival Splendor, and it is also 2 formal nights.
There are two formal nights. This link should answer all your questions. http://www.carnival.com/CMS/FAQs/FAQLanding.aspx
p.s. we will be on the ship at the same time.