To determine the number of electronic shell, first, you need to add up the electrons (the numbers at the superscript or power).

1S\(^2\)2S\(^2\)2P\(^6\)3S\(^2\)3P\(^6\)4S\(^2\) →  2 + 2 + 6 + 2 + 6 + 2 = 20

Now we can represent the element with the atomic number 20 is shells i.e  2,8,8,2

This means first shell is 2, second shell is 8, third shell is 8, and the fourth shell is 2 = In total, there are 4 electron shells.