  Quantum random number generators (QRNG) represent an advanced solution for
randomness generation, essential in every cryptographic applications. In this
context, integrated arrays of single photon detectors have promising
applications as QRNGs based on the spatial detection of photons. For the
employment of QRNGs in Cryptography, it is necessary to have efficient methods
to evaluate the so called quantum min-entropy that corresponds to the amount of
the true extractable quantum randomness from the QRNG. Here we present an
efficient method that allow to estimate the quantum min-entropy for a
multi-detector QRNG. In particular, we will consider a scenario in which an
attacker can control the efficiency of the detectors and knows the emitted
number of photons. Eventually, we apply the method to a QRNG with $10^3$
detectors.
