In 7th edition a CAD is a combined arms detachment. It's the standard 1 HQ and 2 TR minimum force organization chart. In 7th you can take as many CAD as you want. That means as long as you fill 1 HQ and 2 TR requirement twice, you can jump up to 6 heavies, 6 elites, 6 fast attack choices in your army.
The knee jerk reaction is to limit CAD to one per army and go from there with allies, formations, etc...
Now. Mike Brandt over at NOVA is thinking that limiting CAD does nothing. It does not prevent any more abusive armies then what is currently available with just one CAD. Here is his article.
http://whiskey40k.blogspot.com/2014/06/ ... y-bad.html