* Defining "language" is tricky: Do we count dialects as separate languages? What about creoles and pidgins? How about sign languages?
* Continual change: Languages evolve over time, with new ones emerging and others dying out.
* Lack of comprehensive data: Not all languages have been documented or studied, especially in remote communities.
Estimates range widely, from 6,000 to 7,000 languages. Some sources even go as high as 9,000.
It's more important to understand the complexity of language diversity than to focus on a single number.