How Much Dessert Should i Buy?

I would think 1pp at least, as if you were serving cake.

Can you cut them in half if they are that big? At minimum I'd say you need 1/person with 3 cookies /person. As long as they are decent sized cookies.

Will they be handed out by waiters or will they be self serve? Id probably do at least one per person and maybe a few extras.

I had ice cream for my dessert and ended top having to beg people to take home the extra at the end of the night. I had so much left over. I would just order enough for 75-80% of your guests.

Buy enough that every guest has the opportunity to eat at least one of each item. While many people say otherwise, we have seen too many occasions (not all were weddings either) where there will be a spread of items but a very limited amount. For example, there are 100 guests so the couple provides 100 servings of dessert. Which works when it is one type of dessert that is equally offered. But when they have a variety, they limit the quantity to 5 cinnamon rolls, 10 cookies, 6 brownies, 10 bars, etc to get 100 pieces total. One guest will come up and say “the caramel apple cobbler is amazing so make sure to get some!” And then you go up and it’s been cleaned out for the last 30 minutes. That would not happen if there was enough dessert so that everyone gets to try one of each item or choose one dessert and offer enough for each guest. Leftovers can always be sent home with guests as is very common.