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! 