After a few meetings of oPASS this year the numbers I use are 2 slices of pizza and 2 drinks per person, plus about 10%. I order the large pizzas and get a mix of crusts and topping - remember to get vegetarian! In our case we also have one member who is allergic to onions so we make sure we have at least one pizza that fits in that category. For soda it's usually something with caffeine, something without, and bottled water. It's definitely good to have extra drinks available, they can always be used next time.
Our meetings start at 6 pm and we like to try to serve the food at 6:30 so our speaker can start at 6:45 or so. That means we have to place the pizza order before we know how many will be attending. We always ask members to RSVP but in practice I take that number and multiply by 2 and that seems to work out to be close. Ideally we'd move the food to a little later in the evening so that we could order right at 6 pm and we may do that - or may not!
For planning purposes I figure its $5/person for food/soda/cups/ice/napkins/etc. I typically ask sponsors for a little more than that so we can bank a little for the less frequent but still recurring expenses all groups have.