When the rice is almost cooked, after 25-30 minutes depending on the type of rice, it must still have a bite inside, add the prawns and just let them steep. this takes a maximum of 2 minutes. Fold in the butter and parmesan.
The amount of liquid depends on the type of rice. So it is possible that you need a little more or a little less. The risotto should be quite runny at the end.