But if you mean calculation of no of cells and area of electrodes to be used in PEM fuel cell stack, you can do the required calculations using your current and power requirements.
If the design criteria is for Power Rating, its basically the V and I. You can increase the V by adding up the cells in Series and if you need the I to be increased then active area of the electrodes need to be increased. The design also involves geometry, type of material, Fuel, humidification and the specific Application.