Esperanto
There are people who speak Esperanto in many countries and in all the major continents. No one knows exactly how many people now speak Esperanto in the world. Most sources say that there are between several hundred thousand and two million Esperanto speakers.  A few people grew up speaking Esperanto as their first language.  There may perhaps be around 2,000 of these people. Therefore, Esperanto is the most-used constructed language in the world.