* Defining "written language": What counts as a distinct written language? Are dialects with different writing systems considered separate? What about historical scripts that are no longer used?
* Ongoing evolution: New writing systems are constantly being developed, and some existing ones are being adapted or abandoned.
* Lack of comprehensive documentation: Many languages, especially those spoken by smaller communities, may not have been thoroughly documented or researched.
Estimates range widely, from around 3,000 to 7,000. However, it's important to remember that these are just estimations, and the actual number is likely somewhere within that range.