On my next Catering job, I was asked to cook a large amount of rice. I will probably use about 15 to 20 cups of Basmatti or Risotto rice.    For the Basmati, my options are to either cook the rice 4 cups at a time (re-season, clean the pots, etc) or use my large soup / gravy pot to do the rice.  For the Risotto, I'm not really sure if cooking that much risotto is even possible simultaneously.   My question in a nutshell is how much water and salt should I put in?  Now I know that water and seasoning depends on humidity, weather condition, type of rice, where was it harvested from, was it drained ? washed ? drained and washed? soaked?, the moon and earth cycles, night and day, and that the amount of water decreases significantly as the amount of rice increases. I'm looking for a ballpark figure (+/- 1 cup)  EDIT  I'm sorry to be a pain in the rear; however, there is definitely a difference between cooking 2 cups of rice and 20 cups of rice. I ended up resorting to making my spice mix ahead of time and cooked the rice 4 cups at a time with about (5-6 cups of water for each batch).   I'll keep researching this and see what I can come up with 
One great way to cook rice is the iranian style chelow or polo. Rice is soaked in salted water for a couple of hours then it is boiled the same way as pasta 'al dente'. Finally it is drained then steamed for about one hour. The question of how much water doesn't hold. 