Small portable devices carried by users can be exploited to provide useful information through active cooperation. To guarantee privacy, the system should use the information on the device without tracing the user. In this work we address this scenario and we present a privacy-preserving distributed technique to estimate the number of distinct mobile users in a given area. This could be of crucial importance in critical situations, like overcrowded airports or demonstrations where monitoring the number of persons could help organizing emergency countermeasures. In our envisioned scenario users periodically transmit a sketch (i.e. a summary) of the users they met in the past obtained applying suitable, duplicate-insensitive hash functions. Sketches are based on hash functions, so they also provide some kind of privacy, as it is not possible to hardware to show that it works on real devices.