The number of handicapped spaces is a simply ratio to how many total spaces are in the location. I don't remember off the top of my head what that ratio is.
I want to say it's in the vicinity of 1 in 40. My memory might be faulty, though.
ETA:
http://www.ehow.com/about_5171795_requirements-handicap-parking.html
This says 1 in 25 for small lots, 1 in 50 for lots 500-1000 spaces and then a formula of roughly 2% after than. Details are in the link.