Why the last level stops at eight
Look again at potassium and calcium in the table. Something there needs explaining.
The third level can hold up to 18 electrons. So why is potassium written 2, 8, 8, 1 rather than 2, 8, 9? And calcium 2, 8, 8, 2 rather than 2, 8, 10?
The rule for the last level
When a level is the last one holding electrons in an atom, the maximum it can hold is 8.
That is why potassium's ninth electron in the M level does not appear there. With 8 already in the third level, the level is full as an outermost level, and the next electron starts the fourth level instead.
The 18 is not wrong — the third level really can hold 18. But it only does so once it is no longer the outermost level, which happens further along the periodic table than the twenty elements you study here.
So, for elements 1 to 20
K — up to 2
L — up to 8
M — up to 8 while it is outermost
N — begins once M holds 8
Keep this rule close. The number of electrons in that last level is what decides an element's chemistry — and in section 3.3 it is what decides where the element sits in the periodic table.