How much dessert?

I say about 3 pieces per person.

Make sure there is enough of each item for each guest to have at least one serving. There is nothing worse than another guest coming up to say “the ___ was awesome! You have to try it!” to see there was only enough for 10 of 100 guests. You can always send home leftovers with guests in take out boxes or donate to a local shelter.

Hmm will the guests be serving themselves or will it be brought the table. If brought to the tables I d do one whoopie pie and 2 cookies each. If its mores of a self serve option I d try to have at least 100 whoopie pies (some people may not want one while others will most likely take more than 1 and 3 cookies a person. You can also have little boxes / bags to send home with people if there are extras. Try to have a few set aside for you your partner for later it ll be nice to nibble on afterwards.