electronic configuration - Do atoms react to fill outer shell or 8 valence electrons?

I recall in middleshool chemistry we simply said electron shell configurations were 2,8,8,many which I would have no problems with.

But now I'm learning that shells contain $2n^2$ electrons where n is the shell number.

Then why is "full outer shell" and "8 valence electrons" still used interchangeably when describing stability? i.e we are taught that atoms react in ways that achieve an outer shell of 8 valence electrons. But at the same time, they react to form full outer shells.