I've read that between 3000 and 6000 languages are spoken on Earth. My question is the following: how do people calculate that number?
The Ethnologue is a common source of reference for language classification and documentation. It uses the ISO 639-3 standard, which classifies languages using a three-letter coding system. The basic criteria that Ethnologue uses to identify languages (as opposed to, for example, dialects) are (i) mutual intelligibility between speakers of variants of a language; and (ii) existence of a common literature or of a common ethnolinguistic identity. The criterion for including a language in the ethnologue is that it must be “known to have living speakers who learned [it] by transmission from parent to child as the primary language of day-to-day communication”.