How can a shell in an atom store more than 8 electrons ?

The number of electrons in each shell is determined by working your way through (very complex) calculations that begin with the Schroedinger equation (which we will not investigate here).

As you go through the solution of this equation, you find many solutions, which correspond to spacial descriptions of the possible behaviour of electrons in atoms. These solutions are known as orbitals.

These descriptions involve four values known as quantum numbers. The first of these is called the principal quantum number, #n#. It determines the shell in which an electron will be located.

To make a very long story a bit shorter, I will point out that in general, for any value of #n#, there are #n^2# possible orbitals.

Add to this the fact that any orbital call hold two electrons, and you get this result:

The first shell (#n=1#) will hold #1^2 xx 2 = 2# electrons.

The second shell (#n=2#) will hold #2^2xx2=8# electrons.

The third shell (#n=3#) will hold #3^2 xx 2 = 18# electrons.

and so, following the same pattern.

So, the concept of a shell is not at all limited to 8 electrons. In fact, this is the maximum number of electrons only for the second shell!
A shell with a high value of #n# has a great any orbitals within it, and can hold many electrons.

Now, why that still allows that the number of valence electrons will reach a maximum of eight is another story!