10 million people or so live in Tokyo. This city is so big, that in fact it can be thought as the collection of many smaller cities, one for each metro station. By the way, remember to carry with you an up-to-date map of the intricate Tokyo metro network.
When I travel for businnes in Tokyo, I always take with me my camera: it is such a huge city that one should never miss a chance for taking a new souvenir.