8 cups of grits to 32 cups of water and the other accoutrements as you wish says All Recipe's catering calculator for 50 people. I use it a lot and it's pretty good. It seems obvious it's 4 to 1 with the liquid so even 10 cups of grits to 40 cups water would be reasonable, I'd think.
If you want cheesy grits, they add 8 cups of shredded cheddar too!
