Simple ! it's carbon atomic number 6 . it's atomic mass 12.3..!

how to calculate
6 + 6 is equal to 12 and it's nearly 12.3
and of Chlorine
no it won't match to your question

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It is not always possible to calculate the atomic mass easily by knowing the atomic number of an element.

atomic number = Z = number of protons or electrons.
atomic mass = mass of protons  + mass of  neutrons  in an atom
                    ≈  number of protons + neutrons
Actually neutrons have mass a little more than 1 proton.

For large atoms with high atomic number, the atomic mass is more than the sum of their numbers.  Also the number of neutrons is more than the number of protons in large atoms.

Perhaps there is an empirical relation between atomic number and atomic mass.

atomic mass = 2 * atomic number  for small atoms  (for carbon or smaller)
atomic mass > 2 * atomic number for large atoms.

 Element               Z     mass/Z

Hydrogen :  H2 :     1        2
Boron        B          5      2.16
Carbon       C         6      2.15
Sodium     Na       11      2.09
Oxygen:     O2      16      1.9999
Chlorine     Cl        17      2.08
Iron            Fe:      25      2.15
Silver         Ag :     42      2.28
                 W       74       2.49
Polonium              84       2.5
Uranium :   U:       92       2.59

The graph between  Z and  mass m  looks like a parabola curving upwards  with central axis being the Y-axis.  You can draw this graph and then for a given value, you can find that from the graph.

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but it is possible in some casess
I guess a graph like this only can help... perhaps there is an expression for mass M in terms of Z..
perhaps in some cases, it is possible to estimate..
what i said is possible in some atoms