We're having trays of sandwiches at our reception, which is at a nonmeal time. Per the deli, each tray is 20 sandwiches, which feeds 20 people. The sandwiches are cut in half, so 40 pieces per tray.
We have 43 guests coming, 13 of which are kids under 10. Should I get two trays or 3? I'm thinking three trays would equal a lot of uneaten sandwiches, since the kids won't eat as much as an adult would. If the kids ate half a sandwich each and adults ate a whole, that would be 67 pieces needed, or two trays. We are serving other snack type foods as well: chips and dip, cheese and crackers, fruit, and cake.
Re: Calculating servings
This reminds me of one of those crazy word problems in 10th grade math.

Get 3 trays, but have one of them meatless. More is always better, and adding the third tray wouldn't cause an absolutely unreasonable amount of extra food.
For example, One tray of turkey or chicken, one tray of ham or roast beef, and one plain cheese/veggie. Problem solved.
Even if you don't have vegetarians on your guest list, a lot of people (myself included) like a meatless sandwich anyways.
I really, really like the idea in PP to have one of them veggie!
I like the sound of your reception -- reminds me of the weddings I grew up attending.