I understand what you mean, but technically a letter is a symbol that represents sounds in an alphabet (alphabets ideally represent individual consonants and vowels).
Other types of scripts that represent sounds often do not use the term letter, but symbol instead.
Mandarin uses hanzi (aka Chinese characters). Character is the preferred term (although characters and letters are types of symbols). Mandarin characters represent meanings (and often not an entire word). Many words require at least two characters.
4000 are recommended for daily literacy, but there are at least many tens of thousands, and some say more than 100,000. But many of those are no longer used.
There are also two styles: traditional and simplified. The answer also depends on whether or not you consider simplified ones to be different characters.
If we go by a strict definition of letter, then Mandarin has none.