At the time I was elected (1969); there was a quota based on the number of registered boys in the troop, not the number eligible for election. The chart in the OA Handbook ranged from 1 elected for a troop of 5 or less, to 10 who could be elected by a troop with between 74 - 85 boys.
There was also a provision for electing Eagle Scouts that were not member, that would not count against the troop quota.
The one thing that kind of caught me by surprise, as I really did not remember this, was that you did not have to be First Class to be elected. A boy could be elected as long as he attained the First Class rank within 6 months, and prior to induction. (from the 1970 printing of the handbook) In looking at my 1975 60th anniversary copy, the requirement was by then that a Scout must be First Class to be eligible for election.