Depends on targeting. If you want to try to WIN, you will use a counter force targeting strategy where you shoot for the military and industrial assets of the other nation. Military installations, ports, railroad centers, oil production facilities, major factory areas. That would cause millions of deaths but not as much as the alternative, counter value. That is where you are firing mainly to destroy the other nation completely. You target the major population centers. Many more will die but since you left the military assets intact, they could still effectively strike back. It is all moot, if you fire off 25 or more nukes it is possible the dust raised would give us a nuclear winter where no crops would be harvested for a year, maybe two. If you do not have food stored up AND the means to grow more immediately after winter, you are screwed. How many Americans have a year supply of food? How many can grow food on their own? Most Americans do not have the food or the ability and they would simply starve.