Why not try more?
Digging and dividing dahlias is hard work! If they do freeze, replacements can be easily bought. How much is your time worth?
Some hints: Be sure to mulch the plants with several inches of compost or dry leaves. Make sure they are not in a wet area as they will rot in cold, wet soil. Consider digging them in the Spring every few years and dividing the large clumps. As clumps get too large, they send up too many sprouts and the plants will become too crowded an be spindly. Be sure to control slugs and snails in the Spring as they will lay eggs on the clumps and the hatchlings will eat the sprouts before they come up. By the way, you can easily move clumps around in the Spring and they will do fine when re-planted.