Question

Why carbon has atomic no. 6

Answer 1

Atoms of the same element and same atomic number can have different numbers of neutrons. All carbon atoms have 6 protons. Most carbon atoms also have 6 neutrons, but some carbon atoms have 7 or even 8 neutrons. Scientists call these different kinds of carbon atoms "isotopes" of carbon.
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Answer 2

An atom's atomic number is determined by the atom's number of protons.
Carbon has 6 protons in its atomic structure, hence the number 6 for its atomic number as well.