converting cups of rice to kilos
If you cook rice for 200 people you could consider 100 grams per person so a total of 20kg.
I'm making a lot of assumptions here, I know, like I have no idea what your other servings look like.

As a general piece of advice and as semi-professional chef, I'd recommend to use grams at all times. Cups is a volume-based measurements and rice has a lot of air around it, and its volume will depend on the type of rice, temperature of the environment in which you measure and even humidity! If you go pro, you should resort to using accurate weight-measurements rather than volume-units.