That's not an assumption, it's a fact. There are a finite number of arrangements of protons and neutrons which can yield atoms. Each sequentially increasing cou

That's not an assumption, it's a fact. There are a finite number of arrangements of protons and neutrons which can yield atoms. Each sequentially increasing count of protons is a new element. Adding neutrons to a given count of protons makes an isotope. We know which of these can exist in nature, and which can't due to their short half-lives / instability.