I am making risotto for a Christmas dinner, serving 20 people. I plan to multiple the recipe by 3 or 4x. Since risotto is a challenging dish at the normal scale, how can I ensure getting good results when multiplying the recipe?
If you haven't done it before don't do this for Xmas day, it's not worth the risk. Always practise on less important occasions
You need a wider, rather than taller pot. Nearly a paella style pan. And a strong arm
Ingredients wise, I don't know of any magic scaling tricks for risotto
The hardest part is stirring enough, but not too much, and doing an even job of it
Probably easier to have two pots going at the same time, rather the one large pot. Then you can have two different flavours going!
You don't multiply the recipe. You have to make it multiple times.
Risotto isn't as hard to do as its fame suggests, but it requires a very even heating. If you pile your rice deep, you won't get it right even with constant stirring. You always want a thin layer of rice in the pan. The good news: if you do it correctly, you can leave it there without the constant stirring.
Still, if you haven't cooked it before, I support TFD: Don't try it for the first time with your guests, it has a somewhat high failure rate if you are inexperienced. If you insist on it, make it 1-2 times before as a test.
For best results and easiest preparation, make it Food lab style.
Perhaps the restaurant trick of preparing risotto in advance would be helpful for you?
See this ChowHound thread http://chowhound.chow.com/topics/300522 where the risotto is cooked only halfways, then put into the fridge on sheet pans. This leaves less work with the guest there.
I would be afraid of making risotto for more than 8, perhaps 10 people. In the few attempts at large risotto I have seen, it was frequent that the violent stirring required by such a large mass of rice tended to break the grains (and make glue). Another frequent occurrence was rice forming a burnt layer on the bottom of the pot. If that happens, keep stirring with a light hand, lower the gas and avoid at any cost to scrape the burnt crust into the risotto. It will taste bitter and spoil the whole thing.
But yeah, do two pots, it is good advice.