Following the order: KLMN, N is fourth shell of an atom. The number of electrons in a shell depends on the element, its oxidation state and its electronic state. Considering a electrically neutral atom in the ground state, the elements in H-Ar range does have no electron in the N shell (K, L and M shells are being filled).
K is the first element where N shell begins to be filled ([Ar]4s¹ configuration) followed by Ca ([Ar]4s²). The next elements are transition metals (Sc-Zn) which, owing to the filling of 3d orbitals, does not have its N shell changed (except by a couple of exceptions). After Zn, in Ga-Kr range, N shell is filled with 3rd to 8th electrons which occupy 4p orbitals. The next two elements, Rb and Sr still have 8 electron in N shell because 5s orbitals are filled. Then 4d orbitals are filled across Y-Cd transition metals, growing N shell from 9 to 18 electrons. In the next elements (In-Xe) 5p orbitals are filled and N shell remains unchanged, the same happening to Cs and Ba where 6s orbitals are filled.
The N shell are filled again in the lanthanoid series (La-Yb) where 4f orbitals are occupied. In the last element of this series (Yb) N shell is full, with 32 electrons, and no further electron can be added to this shell.