Languages like Chinese, technically, do not use an alphabet but have an ideographic writing system. There are thousands of symbols (pictographs) in Chinese representing different words, syllables and concepts. Chinese Pinyin doesn’t use all of the letters in the Roman alphabet., although there are sounds that don’t exist in English.
The word alphabet refers to writing systems were each symbol represents one sound only, such as our own alphabet (with 26 letters).
The language with the most letters is Khmer (Cambodian), with 74 (including some without any current use). According to Guinness Book of World Records, 1995, the Khmer alphabet is the largest alphabet in the world. It consists of 33 consonants, 23 vowels and 12 independent vowels.