I'd take one of the tiny cups, fill in with water and then tip out the water into a measuring jug to see how much the cup holds. Times this by 50 and you will know how much risotto you need to make. The Thermomix and the Thermoserver will hold about 2 litres of risotto (1 batch).
Personally I think risotto is best freshly made and doesn't reheat well. If time permits I would make it close to serving time and keep it warm in your Thermoserver. Others may have different ideas though.