Neutrons, protons, electrons are known as subatomic particles
explain why that is?

Particles that are smaller than the atom are called subatomic particles. The three main subatomic particles that form an atom are protons, neutrons, and electrons. The center of the atom is called the nucleus.

Together, they constitute an atom, so they are called sub-atomic particles, where neutrons & protons reside under the nucleus whereas electrons revolve outside the nucleus 

Hope this helps!

No it doesnt because it can be any number.