The atomicity of argon is:

The atomicity of argon is 1.
Atoms combine to form their own molecules such as $P_4$, $N_2$, $S_8$, and $O_3$. Atomicity is the number of atoms constituting a molecule and compound. 

Argon is monatomic and its atomicity is 1.
Atomicity of $P_4$, $N_2$, $S_8$, and $O_3$ is 4, 2, 8, and 3 respectively.