I get the impression that for a long time the families have rather had a problem to find enough good quality candidates to be made. They wouldn't close the books if they had a long line of super talents. In the early days they needed manpower for things like the Castellamarese war which opened up the families for non-sicilians. If they wanted to, they could make a 1000 men as long as they circulate the names through all the other families and they accept them. So I suppose the maximum is whatever they all agree on.