If an atom has 6 protons 6 neutrons and 6 electrons what is the atomic mass of the atom?

This is Carbon-12 (12C) it has a mass of exactly 12 universal atomic mass units. It serves as the standard for the universal atomic mass scale.

If you look up carbon on the periodic table, you will find that it says 12.011. This difference is because a small amount of carbon has a seventh neutron, which is averaged into the atomic weight given on a periodic table.