At that time of day, with other food too, I would say allow 1 slice of bread per person (1/2 a round) would be ample..........

From memory, from my hospitality days, we worked on afternoon tea as being "4 bites"

ie 2 quarters of a sandwich, 1 biscuit, 1 slice of cake

Bear in mind, it is not a meal, just a nibble.............

The above may not sound "enough" but this is how we would cater so as not to waste any at all...........it may be that you want to do more, for peace of mind & so that it never looks like it will run out!

HTH