I like to scale both the top thickness and brace height to the overall tension load, so that both the top and the braces take 'their share' of the load. Most 12-string players use a 'light' set. According to D'Addario's catalog they pull about 250# total tension, which is about 50% more than an 'light' set of six strings, or 33% more than a 'medium' set for a six.
Stiffness is the issue, rather than strength per se: any top that is stiff enough to hold up under bridge torque is more than strong enough to avoid breaking any time soon. Stiffness of a plate or beam goes as the cube of height: thickness of the top of height of the braces. To make the top 33% stiffer you only need to make it 10% thicker, and the same goes for brace height. Making the top 26% thicker, and the braces 26% taller, makes the system twice as stiff. For a 12 you will, of course, need a larger bridge and bridge plate, and I like to make the braces wider (say, 5/16" rather than 1/4", to get more glue area), and that adds stiffness as well as weight. Still, the weight doesn't go up as fast as the stiffness.
The power in a plucked string goes as the tension and the square of the amplitude. With a 12 you tend to drop the action height a bit to make things easier on the left hand, and that drops the maximum power available. You don't usually have to reduce it enough to more than offset the tension rise, though.
When I thought of this stuff some years ago I made up a 12 with the top and bracing about 20% thicker/taller than my usual six string (I didn't want to push it the first time out). The top ended up around 25% heavier than a six string for the same size, It was the first guitar I had made that I could not sing over. I saw the customer and his wife at a show a couple of years later and asked him how the guitar was doing. He said it was fine and his wife volunteered that it was loud. "Give 'em volume and they'll hear tone."
