If the 6 proton and 6 neutron of carbon mixed into 12 proton and 12 neutron of magnesium then what the result?

If the atoms fused to produce an atom with 18 protons and 18 neutrons you could get an isotope of Argon. Incidentally the Argon isotope with 18 protons and 18 neutrons happens to be a stable isotope, although not the most common one. You would only get fusion under extreme conditions of course - such as during the detonation of a fusion bomb/warhead (and even then I'm not sure you could get the carbon and magnesium to fuse) or in a star (far more likely) or nova or supernova.

If you simply mixed carbon and magnesium and got them to react you could get Magnesium Carbide (Mg2C).