Quantum numbers of an atom can be defined on the basis of

Each electron in an atom is designated by a set of four quantum numbers. According to Pauli's exclusion principle, no two electron in an atom have same values of all the four quantum numbers. Therefore consequently, an orbital accommodates two electrons with opposite spins, these two electrons have the same value of quantum number ' $n$ ', ' $l$ ' and ' $m$ ' but value of ' $s$ ' will be different.