The atomicity of a substance is the number of atoms that each molecule of the substance has.
To illustrate this with an example, consider the reaction between hydrogen and oxygen to form water.
2H2 + O2 --> 2H2O
Two molecule of hydrogen react with one molecule of oxygen to form two molecules of water. Each molecule of hydrogen has two atoms of hydrogen, the atomicity of hydrogen is two. Similarly each molecule of oxygen contains two atoms of oxygen, the atomicity of oxygen is two. One molecule of water contains two atoms of hydrogen and one atom of oxygen, the atomicity of water is three.
In gases, mono-atomic gases have molecules that are made of only one atom, this is shown by the inert gases. Diatomic gases have two atoms in each molecule, examples are hydrogen, carbon monoxide, chlorine, etc. Triatomic gases have three atoms in a molecule, examples are ozone, nitrogen dioxide, carbon dioxide, etc.