Japan has very few Christians.
The current Wikipedia article Religion in Japan (section Christianity) gives the figure of 0.8% of the population. However, the Wikipedia is quite unreliable, and particularly so on these kind of subjects where some cranks around the world feel obligated to add their own opinions. However, looking at the more reliable The CIA World Factbook, its estimate is very close to the Wikipedia number at 0.7%. (From what I have seen, I would say that even 0.7% seems high to me. I suspect those numbers probably include ethnic Koreans and other non-Japanese segments of the population.)
Also, the previously mentioned Wikipedia article states that most Japanese Christians are Protestant. From what I have seen, that statement seems accurate to me although no source is given for that statement. Another Wikipedia article, Roman Catholicism in Japan, puts the number of Catholics in Japan at 500,000.
A better source of information would be the Catholic Bishops' Conference of Japan, but while their website has a very interesting history of Catholicism in Japan here, they don't seem to have any numbers for the number of Catholics in Japan.
The largest concentration of Catholics in Japan had been the city of Nagasaki, in 1945. (Nagasaki was from where the first Catholic missions to Japan began) The largest Catholic Cathedral in Japan—built for Japan’s largest Catholic population, was “ground zero” for the second atomic bomb, Fat Man. A great number of Japanese whom perished in the attack were Catholic.
“In an instant, 73,000 people died [in Nagasaki], over 8,000 of whom were Christian. Less than one percent of the Japanese population was Christian, yet they comprised over ten percent of the bomb’s victims. A larger issue for the victims, as well as for subsequent generations of Japanese and Christians, was why the West would target the most Christian city in Japan...” (The Journal of Religion and Theater, Vol. 1, No. 1, Fall 2002)