Gherkins, you might be confusing the two different meanings of C12.
Carbon-12 is an atom of carbon. It is made up of a nucleus containing 6 protons and 6 neutrons as well s a cloud of 6 electrons.
Carbon-14 is an atom of carbon (called an isotope because it is the same chemically as Carbon-12 but has a different neclear structure). it is comprised of 6 protons and 8 neutrons as well as a cloud of 6 electrons.
Carbon-12 is the most common isotope of atomic carbon.
Chemical interactions tend to be defined by the number and arrangements of the electrons, and are more or less independent of the nucleus.. Thus the chemical behaviour of carbon-12 is very similar to the chemical behaviour of carbon-14
As we move into chemistry (on a scale a few million times larger than the nucleus), most organic molecules are composed of a string of carbon atoms forming a backbone. Onto this backbone, other atoms arrange themselves to form organic molecules.
Those other atoms include oxygen, nitrogen and above all, hydrogen.
SO if you have a molecule (such as sucrose) of 12 carbon atoms, that would be a C12 compound. (usually the 12 is a subscript).
You would be right in saying that humans as well as other life forms are made up of various molecules based on carbon, in a wondrous variety of combinations.
Other atoms that have this ability to link to themselves include sulphr and silicon. So in theory, it would be possible to create life based on those atoms, rather than carbon. It would be life, but not as we know it, Jim.