  Given a discrete distribution, an interesting problem is to determine the
minimum size of a random sample drawn from this distribution, in order to
observe a given number of different records. This problem is related with many
applied problems, like the Heaps' Law in linguistics and the classical
Coupon-collector's problem. In this note we are able to compute theoretically
the expected size of such a sample and we provide an approximation strategy in
the case of the Mandelbrot distribution.
