The Kana Supplement block spans U+1B000–U+1B0FF, 256 codepoints in Plane 1 (Supplementary Multilingual Plane). Every one of them is assigned in Unicode 16.0.
By script: Hiragana (255), Katakana (1).
By general category: 256 other letters (Lo).
The first 2 arrived in Unicode 6.0 (2010); later additions came in 10.0 (254).
256 characters are East Asian Wide or Fullwidth and take two columns in terminals.
Because it lies outside the BMP, every character here takes four bytes in UTF-8 and a surrogate pair in UTF-16.