A country should only have limits placed upon the number of weapons it has if it is an aggressive country. Japan was unable to have a military after World War II. Iraqis and Afghanis were also limited in what they could do after they were invaded by U.S. forces. Eventually, when nations are peaceful, they should be allowed to have more weapons.
The US has enough nuclear weapons to blow up the earth several times over, and it is not the only country that can make that claim. This is completely unnecessary and dangerous, especially when large quantities of weapons fall into the hands of unstable governments. Already some countries like Russia cannot account for some of their many many weapons that have gone missing.
Limitation on weapons is a good idea but will probably not happen. Most countries use weapons as a means of power, protection and the status of the country. A limitation aims to force equality and less potential dangers in the future but countries can easily lie and create weapons secretly. Even during times of war, weapons will be massed produced so a law like this will probably be a waste of effort and time.
No, I believe that countries should not have a limit on how many weapons they have. First, if there was a proposed limit on weapons, nothing would compel any country to actually follow the rules, especially considering the countries usually pushing for less nuclear weapons, like the United States, usually have a pretty big stash themselves. There is also no way to count how many weapons each country has without complete openness of a country's military weapons stockpile, which would put that country at a considerable security risk.
I do not know how you would ever regulate it. It also depends on what kind of weapons that you are talking about. If you are talking about large weapons of mass destruction than that is something easy to keep track of but if you are referring to all weapons than that is absurd to think that you could track all of them.
Simply put, no matter how much the world works at peace, there will always be war. A country should not be limited in it's ability to protect itself from invasion and attacks from a foreign power. It only takes one country to throw the "limitation agreement" to the wind and start producing weapons again before you have a new super power.