Abstract
The asymptotic form of the average probability to count N indistinguishable identical particles in a small number r ≪ N of binned-together output ports of a M-port Haar-random unitary network, proposed recently in \textit{Scientific Reports} \textbf{7}, 31 (2017) in a heuristic manner with some numerical confirmation, is presented with the mathematical rigor and generalized to an arbitrary (mixed) input state of N indistinguishable particles. It is shown that, both in the classical (distinguishable particles) and quantum (indistinguishable particles) cases, the average counting probability into r output bins factorizes into a product of r-1 counting probabilities into two bins. This fact relates the asymptotic Gaussian law to the de Moivre-Laplace theorem in the classical case and similarly in the quantum case where an analogous theorem can be stated. The results have applications to the setups where randomness plays a key role, such as the multiphoton propagation in disordered media and the scattershot Boson Sampling.