Ubuntu is a Debian-based Linux operating system and distribution, with Unity as its default desktop environment for personal computers including smartphones in later versions.
It is based on free software and named after the Southern African philosophy of ubuntu (literally, "human-ness"), which often is translated as "humanity towards others" or "the belief in a universal bond of sharing that connects all humanity"
Because of a lack of registration, any number provided for Ubuntu usage can only be estimated. In 2015, Canonical's Ubuntu Insights page stated "Ubuntu now has over 40 million desktop users and counting"
Source: Wikipedia