In a multi-robot system (MRS) several robots can operate simultaneously on the same product. We propose to use Timed Petri Nets to evaluate the efficiency of the system. The throughput referred to a single robot-platform and a null loading-unloading time allows us to evaluate the benefits of the MRS. A methodology has been developed that, given a set of robots and platforms, allows to decide what the optimum operation mode is (configuration design criteria) under different conditions of operative configuration, number of subtasks, degree of interference, etc.