Ah,a nearby market is a definite plus! As far as rice goes, the instant/minute rice I'm used to using will feed three people with two cups of rice, so...whatever that works out to, lol. However many people I'll be feeding should equal a few less servings than come in the box of rice, presuming the UK feels a serving is a half cup of uncooked rice. (Actually, does your packaging list number of servings? That's part of the Nutritional Information that's required on ours, but I think that's just a US thing...) Anyway, my giant 42oz/2+ lb. box says it has roughly 27 servings, so I'd use almost all of that to feed twenty people? That seems like WAY too much in my experience, but I've never cooked a rice dish for more than four people, so I'm not sure.

Just buy a family-sized box; I'll just be using it all up at once. If it turns out that's not enough, someone can run to the store for a small box more, right? And as far as veggies go, you can't go wrong with celery or water chestnuts, and I could probably throw canned tinned pineapple in, but it depends on what I actually end up making, so don't worry too much about it. It's not like I'm operating off a specific recipe that requires X amount of ____, or anything.