Japan, a country roughly the size of California (but with a population more than three times ours), runs 55 nuclear power reactors at 17 sites. Here's a list of plants from the Japanese power industry. Note that the Fukushima Daiichi site that's the scene of the current nuclear crisis has six reactors on site, with two more scheduled to be built in the next few years.
Where are all those reactors? The International Nuclear Safety Center at the Argonne National Laboratory in suburban Chicago offers this map, complete with data on each nuclear site.
And here's a breakdown of Japan's nukes from another industry group, the World Nuclear Association.
And a little critical perspective in view of current events. Wall Street Journal: Crisis Revives Doubts on Regulation