How many chlorine ions are required to bond with one aluminum ion and why?

3 Chlorine ions are required to bond with one aluminum ion.

In ionic bonds, metals atoms loses all its outermost shell electrons to form a cation. While, non metal atoms gains however many electrons in order to make its outermost electron shell be 8 (or 2 if there's only one shell).

Therefore, form the periodic table, we can see that aluminum has a atomic number of 13, which makes its electron arrangement be 2,8,3. So, in order to form a aluminum ion, an Al atom must lose 3 electrons. On the other hand, Chlorine has a atomic number of 17, which means it has the electron configuration of 2,8,7. It has to gain only 1 electron to have 8 outermost shell electron.

Thereofre, 3 Chlorine atom are required to gain all 3 electrons given out by just 1 aluminum ion.