Carbon has six protons in its nucleus, so its atomic number is six. It also has six neutrons in its nucleus and typically has six electrons as well.
According to Jefferson Lab, carbon is the sixth most common element on earth, and its stability keeps it from breaking down over time. Because of carbon's structural properties and the ability to bond with itself and other atoms easily, it serves as scaffolding for organic elements. As a result, it is considered the fundamental element of organic chemistry and life itself. Silicon has similar properties, and some speculate that silicon-based life forms may be found in some parts of the universe.