What is the ratio of magnessium ions to chloride ions when they form an ionic compound?

Because magnesium(Mg) has 2 valence electrons(ve-) it will lose them and form a 2+ cation. 

Chloride(Cl-): Each Cl atom has 7 ve- and steals 1 electron to complete its octet. Each chloride ion has a -1 charge. 

So, in order to steal balance out the charge of Mg (2+) we need 2 chloride ions ionically bonded to each Magnesium ion. We will form 
 

Therefore, we will have a ratio of 1:2 :.

Make sense? If you have any further questions please comment back. :)