If Bohr's quantisation postulate (angular momentum $= n h / 2 \pi$) is a basic law of nature, it should be equally valid for the case of planetary motion also. Why then do we never speak of quantisation of orbits of planets around the sun?

(N/A) The quantization of angular momentum is a fundamental law of nature, but its effects are only observable at the microscopic scale.
For planetary motion, the angular momentum $(L)$ is extremely large compared to Planck's constant $(h)$.
For example, the angular momentum of the Earth in its orbit is of the order of $10^{70} h$.
According to Bohr's postulate, $L = n(h / 2 \pi)$, which implies $n = 2 \pi L / h$.
Substituting the values, we find that the quantum number $(n)$ is of the order of $10^{70}$.
For such extremely large values of $(n)$, the difference between successive energy levels or angular momentum states is infinitesimally small.
Therefore, the discrete nature of the orbits becomes indistinguishable from a continuous distribution, and we treat planetary motion using classical mechanics.