I would say millions of people think the world is flat. I would guess the vast majority of indigenous peoples in say the Amazon, or parts of Africa, or many people in the Himalayas or rural India likely believe the world is flat. This isn't due to stupidity but rather a lack of education and the fact that whether the world is flat or round has no bearing on their lives. The Internet and globalization is rapidly changing this, but there are probably many more flat earthers than we often assume.
Now how many in the developed world believe the earth is flat? Hopefully a vanishingly small percentage.