An atom of carbon-12 has an atomic mass of 12, not 11. The number after the element symbol represents the atomic mass, which is the sum of the protons and neutrons in the nucleus of the atom. Carbon-12 specifically refers to the isotope of carbon with 6 protons and 6 neutrons.
Since the atomic mass of carbon-12 is 12, it contains 6 protons and 6 neutrons. The number of atoms in a sample depends on the amount of the substance given, typically measured in moles. If you provide the amount of substance in moles, I can help you calculate the number of atoms.