Answer is:
18 electrons.
The principal quantum number (n) is one of four quantum numbers which are assigned to each electron in an atom to describe that electron's state.
The azimuthal quantum number (l) is a quantum number for an atomic orbital that determines its orbital angular momentum and describes the shape of the orbital, l = 0....n-1.
For n = 3, l = 0, 1, 2.
l = 0, s orbital with 2 electron.
l = 1, p orbitals with 6 electrons.
l = 2, d orbitals with 10 electrons.
2 + 6 + 10 = 18 electrons.