In ionic compounds, the number of cations (positively charged ions) and anions (negatively charged ions) that combine in a compound is determined by the charge balance.
For example, sodium (Na) has a charge of +1 because it loses one electron to become stable, while anions like those with a -2 charge gain two electrons to become stable.
To balance the charges, you would need two sodium cations (Na+) to combine with one -2 anion. This ensures that the overall charge of the compound is neutral.
So, in compounds with a -2 anion, you would typically find two sodium cations (Na+) combined with each -2 anion.