i think the most many trader in the world is in the korea as you can see korea has now the biggest stock of bitcoin, though it is too expensive but i know that korea is so smart to trade some coins in the market also they are smart because they just make the bitcoin as a leggal currency
Japan is having more number of trading sites when compare with the others trading in rest of the countries. We know that bitcoin is been accept bitcoin as a legal money few months back. Therefore, they would not be worry about it is illegal in usage.