I realize this is a very old thread so I'm kind of sorry for digging it up, but bitcoin wiki used it as a reference to spread some ridiculous misinformation which I feel needs to be addressed.

Although that should be obvious, let me remind you that Japanese doesn't have a grammatical number at all, so there is simply no such thing as a plural that follows the rules of Japanese grammar. The same base form of noun is always used. The closest thing you could get is -tachi or -ra suffices which are sometimes described as an optional plural markers which you could add to emphasize plurality. But don't even think about it, satoshi-tachi would be understood as “Satoshi and his group” (perhaps as an allusion that Satoshi Nakamoto isn't one person) no matter how you capitalize it.

It is true that there are some Japanese words which denote pluralities that are created by reduplication, but if you want to compare it to English, they are more like collective or mass nouns rather than plurals. In particular you can not use them with a numeral. You can't use hitobito to say something like 5 people.

Secondly, and this is the reason I even care, you got the form horribly wrong. As you can clearly see in examples from Yahoo Answers, the whole word is duplicated. Not first syllable, not first mora, not whatever you had in mind when writing that post above.

So it would be satoshizatoshi.

Now the fun part is that your form sounds like something completely different. Had there existed an adjective satoshii (and satoshii sounds like one), then satoshisa would mean a degree to which something is satoshii, a “satoshiiness” (cf. tsuyoi—strong, tsuyosa—strength; takai—high/tall, takasa—height; ōkii—big/large, ōkisa—size/volume; &&c).