Buffet Catering Questions?

I am doing a finger buffet for around 70 people. I am doing things like sandwiches, sausage rolls, cubes of frittata on sticks but my question is, as it is just a snack buffet at a party and people will have eaten a proper meal earlier, what kind of numbers of sandwiches etc should I do? Should I do 2 sandwiches (i.e. 2 quarters of a whole sandwich) each, a sausage roll each or more or less as not everyone will eat everything! Any help or advice would be great from people who have catered finger food for such a large party. Thanks