Question

If the third shell can have 18 electrons why does it follow the octet rule

Answer 1

Every shell is denoted by the principal quantum number n.n=1,2,3 etc...

Every shell has subshells denoted by azimuthal quantum number l. Where l=0 to n-1. Sub shells are named as s,p,d,f etc...

Every sub shells have a number of orbitals denoted by magnetic quantum number ml
Where ml=-l to +l. Each orbital can hold 2 electrons.

The electrons will be filled in the increasing order of their (n+l)

The third shell have s,p,d sub shells where each can hold 2,6,10 electrons respectively, so a total of 18 in third shell.

What I'm going to say next is my own answer which i found out by checking noble gas configuration as i wasn't able to get a useful answer from any other sources. So the answer may not be correct

But after filling p subshell the 4s subshell will be filled according to the n+l or Aufbau's principal.
So for more electrons to be added it should be in the next shell. Therefore the third is currently having its max number of electrons without going to the next shell hence it's stable.
This is the reason why octet rule works because without going to the next shell any shell can hold electrons only in their s and p sub shells which is 8 electrons. (Octet rule has some exceptions).