Despite the freestyle dining option you wear what you feel comfortable in. There will be some formal evenings plus there will be at least one restaurant where you could dine most evenings with your dinner suit, but they may cost you a little bit extra. I always take a tuxedo with me as I prefer this to a dinner suit and on the last ship there were very few guests of a similar mind, out of 1,900 only about 50 of us, but that didn't bother me or my wife. Nowadays more are going in for a lounge suit or a smart jacket given the freestyle dining option and quite few are of the younger age group.
There is your answer! 50 out of 1900 will make you feel like one of the waiters! If you are going with NCL I wouldn't bother with a DJ.