As far as I know, most (*) restaurant will prepare risotto in advance, will prepare the basic recipe and undercook it a little bit; cool it quickly and portion it put in the fridge/freezer and then when ready to serve, they reheat and add flavouring.
See this :
Is the risotto used for a side dish, look for about 50-ish grams or a main dish 120-ish grams for the rice; do a test in advance to see if you need more or less rice (from what I can find on the internets)
So about 5 KG to 12KG of rice.
As for the cooking vessels, I'd use as large a pan that can fit your cooktop and do multiple small batches, will be easier to put on a pan and cool down.
I'd go for about 500 grams per batch, again, test in advance to see if it fits your pan. You should be able to easily stir the rice and add the liquid.