hi UrGuy,

How they got so many couples... I think they were volunteers.
and how they got all the guys hard at one time... Show them 250 naked girls?
and why it was made... Same reason they make any movie.

I got these sites from google. You could look for more. For your search engine try 'Behind the scenes of japanese film 500 sex', or 'the making of japanese movie 500 sex', or 'how did they make japanese movie 500 sex'. Let us know if you find anything good.

This is what I found...
http://en.wikipedia.org/wiki/500_Person_Sex
http://ethnorotica.com/article/129/...n-japanese-orgy
http://www.moonbuggy.org/archive/20...ple-having-sex/