According to the quantum mechanical model of the atom what is the maximum number of electrons that can occupy each energy level?
Think of it in terms of orbitals (which correspond to the blocks in the periodic table) - s can hold 2, p can hold 6, d up to 10, f up to 14.
Increasing principal quantum numbers (typically referred to as 'shells' in pre-sixteen courses) can have progressively more orbitals e.g. one has only s (2 electrons in total), 2 has s and p (8 max), 3 has s,p and d (18 max), etc. This is all part of basic quantum theory.