Contrary to popular myth, risotto can be cooked up to a point, up to an hour ahead, and the last batch of stock added just before serving. The trick is to cool the risotto quickly, so that stops cooking.
When you are ready to serve, carefully reheat the rice, and add the last remaining 150ml of hot stock, mixing well. When this has been absorbed, add the finishing ingredients in the normal way.
As a general rule, allow 100g of uncooked rice per person, and up to 230ml of total liquid per 100g of rice (this might be made up with hot stock, or a combination of dry white wine and hot stock).
The total amount of liquid needed varies with the age and adsorption qualities of the rice - so much less might be needed, but if you have this quantity hot and ready you can't go wrong. It is important to then only add it a ladle at a time.