How much Finger Food to make for a 'crowd'
This is the rule we use for kids parties in Brazil, where pretty much all that is eaten is finger food (besides the birthday cake), you can use it as a starting point for other regions as well (since portions are typically larger in the US I'd increase it a little):


6-8 snacks (savory) items per adult, 4-6 per child
4-6 sweet items per adult, ~4 per child (who will eat most of the cake)
Lots of drinks (soda, juice, alcohol for adults), which I don't remember the amount


A savory item is roughly the size of a 1" / 2.5cm diameter ball, while the sweet ones are a little smaller (1/2" / 1cm diameter).

With those quantities nobody ends up completely stuffed, and there isn't much leftover either. For kinds of food which I can reuse later I tend to get a little more than the "recommended" amounts, since I'm able to use the leftovers the following day.