How many rings or shells can an atom have?

Answer 1

A lot. Just because an electron (or more) isn't out there does not mean the shell does not exist. A shell is, if you recall, an energy level. A given atom has a given number of occupied shells in its neutral, un-ionized state. When an electron in any atom is given energy (the right amount), it can skip up to another energy level. The atom has become ionized; the electron had gone to a higher energy level. The electron then releases the energy and falls back. This ionization and de-ionization process happens continuously in gas discharge lighting (think fluorescent lights). And more applied voltage will kick electrons up to even higher energy levels. It gets more complicated. When a bunch of atoms of something are around, they have the individual "possible energy levels" as mentioned. (They're actually called Fermi energy levels.) But when some atoms are clustered together, other energy levels can exist where electronis may go that did not exist in the individual atoms. In compounds, its even more complicated. Yes, there will be a "practical upper limit" to the number of energy levels possible because at some high energy, the electronis are all torn away and the naked nucleus is left exposed. This is what happens in heavy ion colliders when atoms are stripped of their electrons and the nucleus is put into the bore of the accelerator. But those energy levels did and do exist, whether electrons are in them or not.

Answer 2

An atom can have multiple rings or shells, depending on the number of electrons it possesses. The number of rings or shells an atom has corresponds to the energy levels occupied by its electrons. The larger the atom, the more rings or shells it can have.