The number of atoms constituting a molecule is known as its atomicity.
Atomicity is monoatomic if there is only one atom present, diatomic if there are two atoms present, triatomic if there are three atoms present, and so on.
If there are more than four atoms present, the atomicity is polyatomic.
Atomicity of sodium
Sodium is made up of only one atom.
So, the atomicity of sodium is as it is monoatomic.