Atomicity of a molecule refers to the total number of atoms that constitute that molecule.
Fe(HCO3)3 has iron, carbon, hydrogen and oxygen atoms. It indicates that there are three (HCO3) ions and one Fe ion.
(HCO3)3 has 3 carbon, 3 hydrogen and 9 oxygen atoms. Adding all of them together along with 1 iron atom gives a total of 16 atoms. Hence, atomicity of (HCO3)3 is 16.