A circular coil of radius $9 \mathrm{~cm}$ carrying a current of $2 \mathrm{~A}$ is free to rotate about an axis in its plane perpendicular to an external magnetic field of $\pi \times 10^{-2} \mathrm{~T}$. When the coil is turned slightly and released, it oscillates about its stable equilibrium with a time period of $\frac{1}{3} \mathrm{~s}$. If the moment of inertia of the coil about its axis of rotation is $9 \times 10^{-5} \mathrm{kgm}^2$, the number of turns of the coil is ____