Same rules as for a person--enough to reasonably defend yourself with. Just as carrying around ten broadswords on his person would be too much for a single knight, nations have to use their best judgment as to how much is too much. Enough to destroy the planet is far, far too much.
The larger powers have been reducing their numbers of nuclear weapons for years, now. What we have to worry about these days are countries with small numbers of nuclear weapons, with despots in power.