How do they determine how many handicap spots a parking lot is supposed to have? Is it supposed to be a percentage of total parking available?

I was at a baseball game a few years ago, and there were Huge empty LOTS full of the blue signs right next to Comiskey Park. I'm talking Hundreds of parking spots with very few cars in them. Crazy waste of space, but I'm sure there's some law that says they have to have that many...