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The atomic number is ALWAYS equal to the number of protons; the mass number is equal to the number of protons & neutrons combined.
Element 6 which is carbon has several isotopes (forms of an element with varying numbers of neutrons).
The most common isotope of carbon is carbon-12 which has 6 protons 6 neutrons and 6 electrons. Its mass number is 12 (6 protons + 6 neutrons = 12). Since electrons are so tiny their mass is generally disregarded (think about it like this: if a 12 lb. dog has 6 fleas on it how much does the dog weigh? Basically 12 lbs).
Another isotope of carbon which seems to be the one you are referencing has 6 protons 8 neutrons and 6 electrons. This is called carbon-14 and its mass number is 14.
The number of electrons stays the same because adding or subtracting electrons does not change the atom’s mass- it changes the atom’s charge making it either positive (by subtracting electrons) or negative (by adding electrons). These are called ions. Atoms that have a greater number of electrons than protons are called anions and have a negative charge. Atoms with fewer electrons than protons are called cations and have a positive charge. Hope this helps!