Per Ethnologue, there are 7159 distinct languages in the world. While there is no exact definition of a language, there are two main criteria that linguists use to separate the thousands spoken globally. First is mutual intelligibility, or the ability for two people to understand each other. Second is the language's political association (country that the language is tied to). Beyond these factors, languages might also be distinguished by writing system, vocab, and grammar.
Similar languages are grouped into language families and subfamilies. This is also how they are organized on protocol. If you know one language in a family, it will be easier to learn another language in the same family than it will be to learn a language in a different family. Closely related languages are also more likely to be mutually intelligible.