Pizza that is baked with cheese and sauce can be considered a form of pas haba bekisnin, bread that has pockets. The gemara says that such a food is unique, in that it requires a bracha of mezonos/al hamichya, unless one eats enough of it that is a shiur ot “kevias seudah”, the amount of food one eats during a meal. Even if he eats other things aside from the PHB”BK, they combine to fill this measurement.
There are 3 different opinions as to what defines the term “phb”k”. Some hold that it’s any bread-like dough that’s sweetened (a cake), others hold that it’s a pie type of food with a filling(hence the term”kisnin”, pockets), and a minority view is that it’s brittle(like pretzels). We rely on all 3 opinions independent of one another to make mezonos and not hamotzi, unless one has enough of it (or some of it with other food to combine) to have a meal. The Mishna berurah says that this amount is “3 or 4 bayim”,, or egg sizes, which is the equivalent of 8 shot glass sizes of food. Counting the “fluff” is a shailoh.
Pizza has a filling and is hence pas haba bekisnin. However if the dough is baked separately, it is definitely hamotzi. A slice has a lot more than this shiur, yet there is an argument to be made that the shiur describes a seudah, and one slice of pizza as it is eaten culturally isn’t a meal. We may also rely on the stricter opinions for kezayis and say that the slice doesn’t have 4 bayim. The common minhag is to only make hamotzi on two slices, and rav belsky told me that i personally can rely on this. (The implication was that each person should ask their own shailoh).