This was a 7 day Western Caribbean itinerary. I was told there were about 3000 adults and 1100 children (not sure if that meant under-12 or under-18) which is about nominal capacity (there was definitely quite a bit of 3rd and 4th occupancy). The ship did not seem very crowded, but this was not a holiday sailing. This a large and complex ship, even experienced cruisers will have to spend some time learning its features.
Be sure you understand NCL's "Freestyle" concept before booking. There are also several "reserved" areas on the ship, including the Courtyard Suites restaurant and lounge, and adults-only areas, so this is not entirely a one-class ship. One oddity- much of the furniture is designed for really fat people. However, for a cruise ship, most of the passengers seemed quite fit and healthy, so the furniture was actually rather uncomfortable.
Once you have a cover charge for one restaurant, as almost all ships do, I see no reason you cannot have a charge for others. Nor do I see why the same principle cannot apply to entertainment. However, it can get confusing, and NCL doesn't do a good job of explaining.