There are no letters in the Japanese writing system. Japanese officially uses the following set of symbols, which total a lot more than 204:
Hiragana - 71 symbols, each representing the sound of a syllable.
Katakana - 71 symbols, each representing the sound of a syllable, used mainly to write foreign words.
Kanji - officially there 2,136 symbols, each representing a word or concept. In practice, there are thousands more. Kanji are borrowed from Chinese, which has more than 50,000 symbols.