How many electrons does potassium have in its outside electron shell?

Potassium (K) is of atomic number 19 and has only 1 electron in the outermost shell

And this is because of the octet rule of chemistry followed while determining the number of electrons in each shell.

According to the octet rule, there can only be a maximum of 8 electrons in the outer shell of an atom, So when 'L' shell of K is filled with 8 electrons to accommodate the 19th electron a new shell will be created