Someone launches one or two. They launch three or four in retaliation. Someone's allies get involved and maybe they'll launch one or two or they'll just declare war, then maybe whoever they were against would launch them against the allies.

Then you have all that radioactive dust knocking about being carried by the wind, the dead, the should-be-dead-and-are-on-their-way, the-completely-fucked-for-life and the disease and all the rest of it putting immense strain on medical resources; food would be an issue for most places, the impact to the world economy, the political ramifications, the military ramifications. One or two get lobbed somewhere and what's to stop the dying throes of a country being the act of launching whatever they had at somewhere?

'Apocalypse' or 'truly apocalyptic' doesn't necessarily mean the pure destruction of the planet. Simply destroying civilisation would be apocalyptic enough and it doesn't take entire nuclear arsenals to do that.

The minute nukes start going back and forth, your life will never be the same again regardless of the numbers.

Saying that, I'll eat my bloody hat if anyone is actually stupid enough in this day and age to engage in nuclear war. Meh.