The number has not been consistent throughout its existence. Russians use an alphabet based on the Cyrillic script. The modern Russian alphabet has 33 letters: 21 consonants, ten vowels and two signs.
The Arabic and Simplified Chinese scripts aren't in danger of going anywhere anytime soon, but the same can't be said for Balinese, Mali, Pahawh (or Pahauh) Hmong, and the other 100-some alphabets ...