There is some debate about the number of people inhabiting Tawantinsuyu at its peak, with estimates ranging from as few as 4 million people, to more than 37 million. The reason for these various estimates is that in spite of the fact that the Inca kept excellent census records using their quipu, knowledge of how to read them has been lost, and almost all of them had been destroyed by the Spaniards in the course of their conquest