I read on the internet that taking more than 450mg of magnesium can be harmful or even cause an overdose.
So how come 500mg magnesium pills exist? I have 250mg pills and it says to take 2 a day.
Depends on the person. For most, the magnesium in multivitamins is more than sufficient. If you need to supplement, consult with your doctor. Typically 400-600mg/day is safe, but depending on the form (magnesium citrate is a powerful laxative, for example), you can have varied side effects.