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.

Not exactly what you're after, but I've heard the guideline of 1/2 pound meat (main course) for dinners, then just proportion side courses appropriately. This might give you a rough starting point, which you could just proportion down depending on where you want this to fit between snack or meal.

As others have pointed out, time of day and other activities will affect how much people eat. Type of snacks can have quite a large impact as well; smaller, easier to handle items will go faster. You can even play with presentation to control how much people eat; placing small serving dishes out, and refilling them from a storage container will subconsciously cause people not to take as much, because there doesn't look to be as much.

Finally, your estimated numbers (50-75) is a HUGE gap. That's a 50% increase! With that large a gap, you're either going to have lots of leftovers, or no where near enough food. There's no way to plan for exact. So, plan things you'll like to eat for the next 3 days. ^_^

As for things to make...

Funeral/memorial snacks tend to be cultural. Are there any local foods or religious foods that are expected? Next, are there any favorite foods of the deceased? Of the family? Also watch for cultural taboos, and voluntary dietary restrictions: a meat-heavy spread in memorial of a vegetarian is probably inappropriate.

If you just want random snack items, most any appetizer would work. Look for things which can be prepared, stored, then served, as kitchen space gets to be a premium in the last hours before the event. Dips are nice, and stretch well. Wrapped items (like pigs in blankets, or boregs) are easier for guests to handle. You may even consider a roast of some sort and making mini sandwiches as a good-sized roast would stretch quite a way (hunt down slider buns, or buy/make small rolls). Picnic foods can be another great source of inspiration.