Rules for Determining Each Quantum Number:
n: 
and is an integer
and is an integerl: 
and is an integer
and is an integerm: 
Two electrons can't have the same four quantum numbers because the four quantum numbers include the specification of the spin of an electron, and two electrons in the same orbital can't have the same spin. When electrons spin, they produce a magnetic field; if two positive or negative magnetic fields are next to each other, then they will repel away from each other. Think about when you try to push the positive sides of two magnets together: they try to push away from each other.
