An old timer back in the mid 1970's told me this formula for the minimum number or rubber bands: a minimum of 8 for any plane, and this includes 1/2A. Then you take the weight of the plane if above 4 lbs, round up to the next pound and multiply by 2 to get the minimum number or rubber bands. For example a 4 lb. 2 oz. plane would round up to 5, X 2 = 10 rubber bands. A 7 lb. 9 oz. plane would require 16 rubber bands minimum. Following this rule I've never lost a plane due to rubber band failure, and even with a few bands breaking in flight.

Hogflyer