There are no letters. There are characters. However, if you take the pinyin of those characters there's a limited amount of possible combinations. Those are multiplied by 4 because of the 4 tones. Here's a list of all combinations: http://www.quickmandarin.com/chinesepinyintable/

声母21个 b [p] p [p'] m [m] f [f] d [t] t [t'] n [n] l [l] z [ts] c [ts'] s [s] zh [tʂ] ch [tʂ'] sh [ʂ] r [ʐ] j [tɕ] q [tɕ'] x [ɕ] g [k] k [k'] h [x] 韵母39个 i u ü a ia ua o uo e ê ie üe (s-)i, (sh-)i er ai uai ei uei ao iao ou iou an ian uan üan en in uen ün ang iang uang eng ing ueng ong iong http://baike.baidu.com/view/22308.htm

23个声母，24个韵母 但是这些只是汉字的发音，汉字本身不是像英语那样用字母组成的。 每个汉字都是个独立的。