Why are there two fluoride ions in magnesium fluoride but only one fluoride ion in lithium fluoride

This would be more of a chemistry question. Remember magnesium has a charge of 2+, and would need to hand off its two extra electrons. Fluorine can only take one electron at a time, so there needs to be two fluorines to take one magnesium's 2 electrons. 

With lithium, it has a +1 charge, so it has one extra electron, which it can hand off to just 1 fluorine atom. 

Another way of looking at this is:  + 2 = MgF2  (the charges must balance out to zero)

+  = LiF (the charges balance out to zero)

The charge of an ion with 55 protons and 52 electrons is +3. the reason is because protons are positive, and there are three more than the amount of negative electrons.