What is the relationship between: the number of electrons and the group number that the element belongs the number of shells/ energy levels

You get 8 groups in the periodic table: 1,2,3,4,5,6,7 and 0 (8)

Each group represents the amount of electrons the element has on its outer shell. For example group 2 elements, will have 2 electrons on their outer shell.

Group 0 (or 8) dont react and that is because they have a full shell of 8 electrons.
In contast to this, group 1 elements are the most reactive elements in the whole periodic table. Think about this, the less electrons there are in the outer shell (for this case in group one 1) then the more they will want to be with their friends so they get angry and become so reactive! Of course, if there is a full shell, like in group 0 (8) also known as the noble gasses, then the elements are happy as theyre with their friends so thats why they dont react.

The more shells an element has, the more the electron (s) on the outer shell are away from the nuclous which reduces the energy level.
Pottasium is a very reactive metal in group 1 and has 3 shells.