How many languages in the world are unwritten?

The exact number of unwritten languages is hard to determine. Ethnologue (17th edition) has data to indicate that of the currently listed 7,105 living languages, 3,570 have a developed writing system. We don't always know, however, if the existing writing systems are widely used. That is, while an alphabet may exist there may not be very many people who are literate and actually using the alphabet. We have data to indicate that 696 languages are unwritten. And for the remaining 2,839 languages we have no data.