There are 14,000 nuclear weapons on Earth. The US has about 5.8K, Russia 6.3K, and the rest are distributed between France, China, the UK, Pakistan, India, Israel, and North Korea*.
There are about 4,500 cities with at least 100,000 people. Even if you assume 3 bombs per city (as some are bigger than others), every city on Earth would be destroyed in an all-out global nuclear conflict.