In the KLMN electronic configuration style of writing K denotes the first shell which can hold up to two electrons, L denotes the second shell which can hold up to eight (2 + 6) or 8 electrons, M denotes the third shell which can hold up to eight (2 + 6 + 10) or 18 electrons etc.
The KLMN style of writing only indicates the number of electrons in each principal quantum number, however the spdf electronic configuration style of writing subdivides each shell into its subshells.
For instance the 3rd principal quantum number has s,p and d subshells, wherein s, p and d orbitals can have a maximum of 2, 6 and 10 electrons respectively.