Maurizio,
first of all, we have to say that using FEFLOW in a cluster environment always refers to doing several simulations in parallel (e.g., for scenario analysis or inverse modeling). FEFLOW does not run a single simulation on multiple machines (only shared-memory parallelization so far).
On the licensing side, currently each single machine running FEFLOW needs a separate license. For running parallelized simulations, please contact us directly for licensing options as you will probably not be able to buy a license for each machine.
Peter