It depends upon the size of the addition and its uses as well as what electrical appliances are in there. Post back with what you are doing with the room and what plans you have for electrical. If it is a 500 sqft bedroom run some 12/2wg nm sheathed electrical cable for a general lighting/receptacle circuit or do it in two circuits with one for lights and one for receptacles. A kitchen addition would require a number of separate circuits of various sizes. A bathroom circuit also requires a number of separate circuits of various sizes along with gfci outlets. A window a/c would need its own circuit of a larger size depending upon the amp rating of the a/c. An office would want separate circuits for the computers with lightning protection. You don't want computers on a circuit with flourescent lights. An entertainment center w/tv,vcr,satelite,stero,etc. may want a separate circuit with lightning protection. A washer/dryer/laundry room requires a number of separate circuits of various sizes. The list goes on. If you are talking about running wire for a new subpanel in the new room then that is another issue altogether. Also what size of service entrance and main panel do you have and how many open slots are available for breakers? If all else fails, hire an electrician. He will need the same answers.