Okay, my specialist subject! If you do it my way, you will have more than you need but it will feel just right.
Sandwiches: 4 finger sandwiches per head (you get 4 strips out of 2 slices of bread with the crusts off), plus you want everyone to have the same sandwich offer and have a few extra for greedy people like me. So make 2 2-slice sandwiches each of ham, smoked salmon, cream cheese and cucumber with salt and pepper, and egg mayonnaise.
Scones: 2 mini scones per person, with jam and cream. You'll need 2 tubs of clotted cream and a jar of jam.
Fancy cakes: 3-4 tiny cakes per person. If you want to make these yourself, make a lemon drizzle, some small tartlets and a fruit or chocolate cake, or a pastry eg eclair. Waitrose does nice boxes of tiny cakes if you can't be arsed - you get 9 in a box so get three boxes and eat the leftovers the next day.
Meringues: optional but nice - either macarons (Lidl sometimes has very nice ones frozen for a reasonable price) or tiny homemade pavlovas, or simple meringue swirls (hazelnut is the best kind). 1-2 of these per person - can lose a fancy cake for each one you include.