Mel is so fond of trellising all the plants in the SFG method that I doubt you'll find *any* info on the number of poles in his book. Maybe a trellis and non-square placement of the bean plants?
Did some searchimg on the web and found my answer in a very good article about pole beans. I don't know the rules on linking other sites here, so I won't do that....
But in case anyone is interested, it said 3 stalks per pole.