I went through the same concerns you did about Carnival and am going on the Carnival Destiny for winter break with my three teens 14 16 and 19. We booked late so really had minimal choice. Everyone tells me that the kids will have a fabulous time and I stopped reading all the reviews since they are not consistent. I finally decided to make the decision myself and thanks to the help of CruiseMates most trusted staff.
As far as your comment about maximum number of teens on the cruise, I was told by my TA that some cruise lines will most definitely only sell a maximum number of "kid" spots so there arent't more kids than adults. I did like the fact that carnival has the 25 year minimum to cruise with underaged kids so we shouldn't find a pile of college students on their own on the ship. Hey, we are going winter break and expect tons of kids (after all I am bringing three already)
Good luck in your choice. I will be sure to post my own opinion when we return.