The maximum number of electrons in each shell of an atom can be calculated using the formula 2n^2, where n is the number of the shell.

In this question, we need to determine the maximum number of electrons in the N shell. According to the formula, when n=3 (as N shell is the third shell), the maximum number of electrons will be 2(3^2) = 2(9) = 18.

Therefore, the correct answer is option 3, which states that the maximum number of electrons in the N shell is 18.

Alert - correct answer should be option 3.