What determines the ratio of ions in an ionic compound?

An ionic formula is effectively an empirical formula. Empirical formulae are the simplest ratio of elements in a compound. An ionic formula is the simplest ratio of ions in an ionic compound.

On the other hand, a molecular formula tells us the exact number of atoms within each molecule. This is why many molecular formulae can be simplified into an empirical formula.

NaCl does not mean one sodium ion and one chloride ion; it means for every sodium ion in the salt, there is also one chloride ion.

MgCl2 means that there are two chloride ions for every magnesium ion in the salt The true numbers would be huge, of course, so we write it as the simplest ratio (the same as an empirical formula).