score:6
I presume you're looking specifically for English-language retreats, which narrows your options significantly; Japan may have a lot of temples, but it doesn't have all that many that can run a week-long course in a foreign language. (Even if the vast majority of that course involves complete silence!)
But have no fear, Wikivoyage's Meditation in Japan is here, and suggests the following:
Poking around on the Soto Zen website's list of temples open to foreign visitors may also find something interesting.