The subatomic particle that determines the number of an element is the proton. Here's why:

* Atomic Number: The atomic number of an element is the number of protons in the nucleus of an atom. This number is unique to each element.

* Identity: The number of protons defines what element an atom is. For example, all carbon atoms have 6 protons, all oxygen atoms have 8 protons, and so on.

* Neutrons and Isotopes: While the number of neutrons can vary (creating isotopes of an element), the number of protons remains constant and determines the element's identity.

Let me know if you have any other questions about subatomic particles!