The molar heat capacity at constant volume for a monatomic gas is

For a monatomic gas, $C_{V,\mathrm{m}}=(3/2)R$

In a monoatomic gas, each atom has 3 degrees o freedoms. They are of translation type. They contribute to the heat capacity.
N atoms of a monoatomic gas has 3N degrees of freedom.
Hence,
$\begin{array}{l}{\mathrm{C}}_{\mathrm{V}}=\left(\frac{\mathrm{\partial }\mathrm{U}}{\mathrm{\partial }\mathrm{T}}\right)\\ =\frac{3}{2}{\mathrm{NK}}_{\mathrm{B}}\\ =\frac{3}{2}\mathrm{nR}\end{array}$
The molar heat capacity is $\frac{{\mathrm{C}}_{\mathrm{V},\mathrm{m}}}{\mathrm{n}}=32\mathrm{R}$